Bar code symbol scanning system employing time-division multiplexed laser scanning and signal processing to avoid optical cross-talk and other unwanted light interference

ABSTRACT

A laser scanning system that employs synchronous time-division-multiplexed laser scanning operations and signal processing operations (for bar code detection). A plurality of successive non-overlapping time slots are defined and logically assigned to a unique laser scanning beam and corresponding photosensor. During a given time slot, the laser scanning beam logically assigned thereto is selectively generated (or selectively projected) into the scanning volume while generation (or projection) of the other laser scanning beam is disabled. Moreover, during the given time slot, the photosensor logically assigned thereto is operably coupled to signal processing circuitry that performs bar code detection operations on the data signals derived therefrom while the other photosensor is operably decoupled from such signal processing circuitry. The frequency of time slots logically assigned to a given laser scanning beam and corresponding photosensor is preferably greater than at least two times the highest frequency component expected in the scan data signal received at the photosensor. In other embodiments, the time slots logically assigned to a given laser scanning beam and corresponding photosensor correspond to scanning planes (or scanning plane groups) generated by the given laser scanning beam during revolution of at least one rotating polygonal mirror. In the preferred embodiment, such synchronous time-division-multiplexed laser scanning operations and signal processing operations (for bar code detection) are embodied in a bioptical laser scanning system comprising a plurality of laser scanning stations, each of which produces a plurality of groups of quasi-orthogonal laser scanning planes that are projected within predetermined regions of space contained within a 3-D scanning volume defined between the bottom and side-scanning windows of the system.

RELATED CASES

The present application is a Continuation of U.S. application Ser. No.10/207,731 filed Jul. 26, 2002; which is a Continuation-in-Part (CIP)of: U.S application Ser. No. 10/045,577, filed on Jan. 11, 2002 and U.S.application Ser. No. 10/045,605, filed on Jan. 11, 2002. Each saidpatent application is assigned to and commonly owned by MetrologicInstruments, Inc. of Blackwood, N.J., and is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates generally to laser scanners ofultra-compact design capable of reading bar code symbols inpoint-of-sale (POS) and other demanding scanning environments.

2. Brief Description of the Prior Art

The use of bar code symbols for product and article identification iswell known in the art. Presently, various types of bar code symbolscanners have been developed. In general, these bar code symbol readerscan be classified into two distinct classes.

The first class of bar code symbol reader simultaneously illuminates allof the bars and spaces of a bar code symbol with light of a specificwavelength(s) in order to capture an image thereof for recognition anddecoding purposes. Such scanners are commonly known as CCD scannersbecause they use CCD image detectors to detect images of the bar codesymbols being read.

The second class of bar code symbol reader uses a focused light beam,typically a focused laser beam, to sequentially scan the bars and spacesof a bar code symbol to be read. This type of bar code symbol scanner iscommonly called a “flying spot” scanner as the focused laser beamappears as “a spot of light that flies” across the bar code symbol beingread. In general, laser bar code symbol scanners are sub-classifiedfurther by the type of mechanism used to focus and scan the laser beamacross bar code symbols.

Such flying spot scanners generally employ at least one laser diode, thelight from which is focused and collimated to produce a scanning beam.The scanning beam is directed to a scanning element (such as a rotatingpolygonal mirror or rotating holographic disk), which redirects thescanning beam across a plurality of stationary beam folding mirrors.Light reflected from a bar code label returns to the stationary beamfolding mirrors and scanning element. A light collecting optical elementcollects this returning light and directs it to a photodetector. Theelectrical signals generated by the photodetector are processed todetect and decode bar code symbols therein.

The bar code symbols are formed from bars or elements typicallyrectangular in shape with a variety of possible widths. The specificarrangement of elements defines the character represented according to aset of rules and definitions specified by the code or “symbology” used.The relative size of the bars and spaces is determined by the type ofcoding used, as is the actual size of the bars and spaces. The number ofcharacters per inch represented by the bar code symbol is referred to asthe density of the symbol. To encode a desired sequence of characters, acollection of element arrangements are concatenated together to form thecomplete bar code symbol, with each character being represented by itsown corresponding group of elements. In some symbologies, a unique“start” and “stop” character is used to indicate when the bar codebegins and ends. A number of different bar code symbologies exist,including UPC Symbologies, EAN Symbologies, Code 39, Code 128, Code 93,Codabar and Interleaved 2 of 5, etc.

In order to produce a successful scan, an object's bar code symbol mustbe oriented with respect to a given scanning beam so that the angletherebetween is not so oblique so as to cause an insufficient amount ofreflected light to return back to the scanner. Therefore, to achieve asuccessful scan, the bar code symbol must be positioned sufficientlyclose to this desired orientation for the given scanning beam.

Thus, to improve the performance of such optical bar code scanners,modern scanners have been developed that employ aggressive scan patterns(i.e., a large number of scanning beams that project into a scan volumeat different orientations), which enable such scanners to successfullyscan bar code labels over a large number of orientations therebyproviding increased scanning throughput. Such modem optical scanners mayemit light through a single aperture (such as a horizontal or verticalaperture) or through multiple apertures. Modem optical scanners thatemit a large number of scan lines through both a horizontal and verticalaperture are commonly referred to as bioptical scanners. Examples ofpolygon-based bioptical laser scanning systems are disclosed in U.S.Pat. No. 4,229,588 and U.S. Pat. No. 4,652,732, assigned to NCR, Inc.,each incorporated herein by reference in its entirety. In general,bioptical laser scanning systems are generally more aggressive thatconventional single scanning window systems. For this reason, biopticalscanning systems are often deployed in demanding retail environments,such as supermarkets and high-volume department stores, where highcheck-out throughput is critical to achieving store profitability andcustomer satisfaction.

Such modem high performance optical scanners typically scan multiplescanning beams though the scanning volume and employ a correspondingnumber of photodetectors for detecting reflection from the multiplescanning beams. In such systems, optical crosstalk degrades performance.More specifically, optical crosstalk occurs when light from multiplelaser scanning beams meet at a target (or any surface in the scanningsystem). The light reflected therefrom follows a return path to thecorresponding multiple photodetectors, which creates an undesirablesignal spike in the output of the corresponding multiple photodetectorsthat can make a bar code symbol unreadable at that point.

In addition, light interference from ambient light and unwantedreflections degrades performance. More specifically, ambient light thatis received at a photodetector in addition to reflections (i.e.,scattered light) from laser scanning beams that are not intended to bereceived by the photodetector can create an undesirable signal spike inthe output of the photodetector that can make a bar code symbolunreadable.

Thus, there remains a need in the art for improved optical scanningsystems that scan multiple scanning beams though the scanning volume andemploy a corresponding number of photodetectors for detecting reflectionfrom the multiple scanning beams, which do not suffer from degradedperformance due to optical crosstalk and light interference from ambientlight and unwanted reflections.

OBJECTS AND SUMMARY OF THE PRESENT INVENTION

Accordingly, a primary object of the present invention is to provide anovel laser scanning system which is free of the shortcomings anddrawbacks of prior art laser scanning systems and methodologies.

Another object of the present invention is to provide a laser scanningsystem employing time-division-multiplexed laser scanning operationsutilizing a plurality of multi-direction laser scanning beams inaddition to synchronous time-division-multiplexed signal processingoperations (for bar code detection) in order to avoid optical cross talkand other unwanted light interference.

Another object of the present invention is to provide suchtime-division-multiplexed laser scanning operations and signalprocessing operation by generating timing signals that define time slotslogically assigned to a given laser scanning beam and correspondingphotosensor, selectively generating and/or projecting the laser scanningbeams into the scan volume of the system such that only one laserscanning beam is active (e.g., actively scanning the scan volume of thesystem) during time slots logically assigned to the one laser scanningbeam, and operably coupling/de-coupling the photosensors from signalprocessing circuitry (that performs bar code detection on scan datasignals derived therefrom) such that only one photosensor is operablycoupled to such signal processing circuitry during time slots logicallyassigned to the one photosensor.

Another object of the present invention is to provide suchtime-division-multiplexed laser scanning operations and signalprocessing operation by generating timing signals that define time slotslogically assigned to a given laser light source (e.g., visible laserdiode) and corresponding photosensor, modulating the power levelprovided to the laser light sources such that only one laser lightsource is active (e.g., ON) during time slots logically assigned to theone laser light source, and operably coupling/de-coupling thephotosensors from signal processing circuitry (that performs bar codedetection on scan data signals derived therefrom) such that only onephotosensor is operably coupled to such signal processing circuitryduring time slots logically assigned to the one photosensor.

A further object of the present invention is to provide suchtime-division-multiplexed laser scanning operations wherein frequency oftime slots logically assigned to a given laser scanning beam andcorresponding photosensor is greater than at least two times the highestfrequency component expected in the scan data signal received at thephotosensor.

A further object of the present invention is to provide suchtime-division-multiplexed laser scanning operations wherein time slotslogically assigned to a given laser scanning beam and correspondingphotosensor correspond to scanning planes generated by the given laserscanning beam during revolution of one or more rotating polygonalmirrors.

A further object of the present invention is to provide suchtime-division-multiplexed laser scanning operations wherein time slotslogically assigned to a given laser scanning beam and correspondingphotosensor correspond to scanning plane groups generated by the givenlaser scanning beam during revolution of one or more rotating polygonalmirrors.

Another object of the present invention is to provide suchtime-division-multiplexed laser scanning operations by modulating adynamic current source that supplies current to a given visible laserdiode.

Another object of the present invention is to provide suchtime-division-multiplexed laser scanning operations through modulationof the power level supplied to a given visible laser diode whereby thegiven visible laser diode is turned OFF by operating the given visiblelaser diode at an optical power level near its threshold optical powerlevel, thereby enabling quick turn on of the visible laser diode.

Another object of the present invention is to provide suchtime-division-multiplexed laser scanning operations and signalprocessing operations in a bioptical laser scanning system that scanswith a plurality of pairs of quasi-orthogonal laser scanning planesproduced by at least two rotating polygonal mirrors, wherein a firstrotating polygonal mirror produces laser scanning planes that projectfrom the bottom-scanning window, and wherein a second rotating polygonalmirror produces laser scanning planes that project from theside-scanning window.

Another object of the present invention is to provide suchtime-division-multiplexed laser scanning operations and signalprocessing operations in a bioptical laser scanning system comprising aplurality of laser scanning stations, each of which produces a pluralityof groups of quasi-orthogonal laser scanning planes that are projectedwithin predetermined regions of space contained within a 3-D scanningvolume defined between the bottom and side-scanning windows of thesystem.

Another object of the present invention is to provide suchtime-division-multiplexed laser scanning operations and signalprocessing operations in a bioptical laser scanning system wherein twovisible laser diodes (VLDs) disposed on opposite sides of a rotatingpolygonal mirror are used to create a plurality of groups ofquasi-orthogonal laser scanning planes that project through thebottom-scanning window.

Another object of the present invention is to provide suchtime-division-multiplexed laser scanning operations and signalprocessing operations in a bioptical laser scanning system wherein asingle VLD is used to create the scan pattern projected through theside-scanning window.

Another object of the present invention is to provide suchtime-division-multiplexed laser scanning operations and signalprocessing operations in a bioptical laser scanning system whichgenerates a plurality of quasi-orthogonal laser scanning planes thatproject through the bottom-scanning window and side-scanning window toprovide 360 degrees of scan coverage at a POS station.

Another object of the present invention is to provide suchtime-division-multiplexed laser scanning operations and signalprocessing operations in a bioptical laser scanning system whichgenerates a plurality of horizontal and vertical laser scanning planesthat project from the top of the side-scanning window downward, whichare useful for reading ladder type and picket-fence type bar codesymbols on top-facing surfaces.

Another object of the present invention is to provide suchtime-division-multiplexed laser scanning operations and signalprocessing operations in a bioptical laser scanning system in which anindependent signal processing channel is provided for each laser diodeand light collection/detection subsystem in order to improve the signalprocessing speed of the system.

Another object of the present invention is to provide suchtime-division-multiplexed laser scanning operations and signalprocessing operations in a bioptical laser scanning system which aplurality of signal processors are used for simultaneously processingthe scan data signals produced from each of the photodetectors withinthe laser scanner.

Another object of the present invention is to provide suchtime-division-multiplexed laser scanning operations and signalprocessing operations in a bioptical laser scanning system that providesimproved scan coverage over the volume disposed between the two scanningwindows of the system.

A further object of the present invention is to provide suchtime-division-multiplexed laser scanning operations and signalprocessing operations in a laser scanning system having a scan datasignal processor with improved dynamic range.

A further object of the present invention is to provide suchtime-division-multiplexed laser scanning operations and signalprocessing operations in a laser scanning system having a multi-pathscan data signal processor that employs different operationalcharacteristics (such as different filter cutoff frequencies, peakthresholds, etc) in distinct signal processing paths.

A further object of the present invention is to provide suchtime-division-multiplexed laser scanning operations and signalprocessing operations in a laser scanning system having a multi-pathscan data signal processor that concurrently performs distinct signalprocessing operations that employ different operational characteristics(such as different filter cutoff frequencies, peak thresholds, etc).

A further object of the present invention is to provide suchtime-division-multiplexed laser scanning operations and signalprocessing operations in a laser scanning system employing a scan datasignal processor having a plurality of processing paths each processingthe same data signal derived from the output of a photodetector todetect bar code symbols therein and generate data representing said barcode symbols, wherein the plurality of processing paths have differentoperational characteristics (such as different filter cutofffrequencies, peak thresholds, etc).

These and other objects of the present invention will become apparenthereinafter and in the Claims to Invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more fully understand the Objects of the Present Invention,the following Detailed Description of the Illustrative Embodimentsshould be read in conjunction with the accompanying Figure Drawings inwhich:

FIG. 1A is a pictorial illustration of an exemplary bar code symbolscanning system employing time-division multiplexed laser scanning andsignal processing operations in accordance with the present invention.In the illustrative embodiment shown, two time-division multiplexedlaser scanning beams cyclically scan through the scanning volume fordetecting and decoding bar code labels disposed on surfaces therein.

FIG. 1B is a timing diagram that illustrates the overlap period betweenscan periods for two representative scan lines (or scan line groups)produced by the two laser scanning beams of FIG. 1A. In accordance withthe present invention, time-division multiplexed laser scanningoperations and signal processing operations are performed during thisoverlap period whereby a plurality of successive non-overlapping timeslots (that occur during this overlap period) are logically assigned toa unique laser scanning beam and corresponding photosensor. During agiven time slot, the laser scanning beam logically assigned thereto(e.g., Laser Scanning Beam 1) is selectively generated (or selectivelyprojected) into the scanning volume while generation (or projection) ofthe other laser scanning beam (e.g., Laser Scanning Beam 2) is disabled.Moreover, during the given time slot (or during a portion of the giventime slot), the photosensor logically assigned thereto (e.g., PD1) isoperably coupled to signal processing circuitry that performs bar codedetection operations on the data signals derived therefrom. Furthermore,during the given time slot, the other photosensor (e.g., PD2) isoperably decoupled (e.g., electrically isolated) from signal processingcircuitry that performs bar code detection operations on data signalsderived therefrom. The frequency of such time-division-multiplexed laserscanning operations and corresponding signal processing operations maybe greater than two times the Nyquist frequency (and preferablyperformed at a frequency greater than four times the Nyquist frequency).In such embodiments, when a given photosensor is operably decoupled(e.g., electrically isolated) from bar code detection signal processingcircuitry, such bar code detection signal processing circuitry operateson a sample of the data signals derived from the given photosensorduring the previous time slot logically assigned to the givenphotosensor. Preferably, this sample is stored on a hold capacitor thatis operably coupled to such bar code detection signal processingcircuitry.

FIGS. 1C1 and 1C2 illustrate timing signals that define time slotsduring the overlap period of FIG. 1B; time-division multiplexed laserscanning and signal processing operations are performed (as set forth inthe table of FIG. 1D) during such time slots in accordance with thepresent invention.

FIG. 1D is a table that describes time-division multiplexed laserscanning and signal processing operations performed by the exemplary barcode symbol scanning system of FIG. 1A in accordance with the presentinvention; such time-division multiplexed laser scanning and signalprocessing operations are performed during time slots defined by thetiming signals of FIGS. 1C1 and 1C2.

FIG. 2A is a side view of an illustrative bioptical laser scanningsystem in accordance with the present invention, showing bottom-scanningand side-scanning windows formed with its compact scanner housing.

FIG. 2B is a front view of the illustrative bioptical laser scanningsystem of FIG. 2A.

FIG. 2C is a top view of the illustrative bioptical laser scanningsystem of FIGS. 2A and 2B.

FIG. 2D is a pictorial illustration depicting bottom-facing, top-facing,back-facing, front-facing, left-facing and right-facing surfaces of arectangular shaped article oriented within the scanning volume (disposedbetween the bottom-scanning and side-scanning windows) of theillustrative bioptical laser scanning system in accordance with thepresent invention; FIG. 2D also depicts the orientation of a horizontal(ladder-type) bar code symbol and vertical (picket-fence type) bar codesymbol on exemplary surfaces of the article.

FIG. 2E is a pictorial illustration depicting a normal of a surface andthe “flip-normal” of the surface as used herein.

FIG. 2F is a perspective view of the illustrative bioptical laserscanning system according to the present invention shown installed in aPoint-Of-Sale (POS) retail environment.

FIG. 2G is a perspective view of a wire frame model of portions of thehorizontal section of the illustrative bioptical laser scanning system,including the bottom-scanning window (e.g., horizontal window), firstrotating polygonal mirror PM1, and the first and second scanningstations HST1 and HST2 disposed thereabout, wherein each laser scanningstation includes a set of laser beam folding mirrors disposed about thefirst rotating polygon PM1.

FIG. 2H is a top view of the wire frame model of FIG. 2G.

FIG. 2I is a perspective view of a wire frame model of portions of thehorizontal section of the illustrative bioptical laser scanning system,including the bottom-scanning window 16 (e.g., horizontal window), firstrotating polygonal mirror PM1, and the first and second scanningstations HST1 and HST2 disposed thereabout, wherein each laser scanningstation includes a light collecting/focusing optical element (labeledLC_(HST1) and LC_(HST2)) that collects light from a scan region thatencompasses the outgoing scanning planes and focuses such collectedlight onto a photodetector (labeled PD_(HST1) and PD_(HST2)), whichproduces an electrical signal whose amplitude is proportional to theintensity of light focused thereon. The electrical signal produced bythe photodetector is supplied to analog/digital signal processingcircuitry, associated with the first and second laser scanning stationHST1 and HST2, that process analog and digital scan data signals derivedthere from to perform bar code symbol reading operations. Preferably,the first and second laser scanning stations HST1 and HST2 each includea laser beam production module (not shown) that generates a laserscanning beam (labeled SB1 and SB2) that is directed to a small lightdirecting mirror disposed in the interior of the lightcollecting/focusing element LC_(HST1) and LC_(HST2), respectively, asshown, which redirects the laser scanning beams SB1 and SB2 tocorresponding points of incidence on the first rotating polygonal mirrorPM1.

FIG. 2J is a top view of the wire frame model of FIG. 2I.

FIG. 2K is a perspective view of a wire frame model of portions of thevertical section of the illustrative bioptical laser scanning system,including the side-scanning window (e.g., vertical window), secondrotating polygonal mirror PM2, and the third scanning station VST1disposed thereabout; the third laser scanning station includes a set oflaser beam folding mirrors disposed about the second rotating polygonPM2.

FIG. 2L is a front view of the wire frame model of FIG. 2K.

FIG. 2M is a perspective view of a wire frame model of portions of thevertical section of the illustrative bioptical laser scanning system,including the side-scanning window 18(e.g., vertical window), secondrotating polygonal mirror PM2, and the third scanning station VST1disposed thereabout, wherein the third laser scanning station VST1includes a light collecting/focusing optical element (labeled LC_(VST1))that collects light from a scan region that encompasses the outgoingscanning planes and focuses such collected light onto a photodetector(labeled PD_(VST1)), which produces an electrical signal whose amplitudeis proportional to the intensity of light focused thereon. Theelectrical signal produced by the photodetector is supplied toanalog/digital signal processing circuitry, associated with the thirdlaser scanning station VST1, that processes analog and digital scan datasignals derived there from to perform bar code symbol readingoperations. Preferably, the third laser scanning station VST1 includes alaser beam production module (not shown) that generates a laser scanningbeam SB3 that is directed to a small light directing mirror disposed inthe interior of the light collecting/focusing element LC_(VST1) asshown, which redirects the laser scanning beam SB3 to a point ofincidence on the second rotating polygonal mirror PM2.

FIG. 2N 1 depicts the angle of each facet of the rotating polygonalmirrors PM1 and PM2 with respect to the rotational axis of therespective rotating polygonal mirrors in the illustrative biopticallaser scanning system described herein.

FIG. 2N 2 is a pictorial illustration of the scanning ray patternproduced by the four facets of the first polygonal mirror PM1 inconjunction with the laser beam source provided by the first laserscanning station HST1 in the illustrative bioptical laser scanningsystem. A similar scanning ray pattern is produced by the four facets ofthe first polygonal mirror PM1 in conjunction with the laser beam sourceprovided by the second laser scanning station HST2.

FIG. 2N 3 is a pictorial illustration of the scanning ray patternproduced by the four facets of the second polygonal mirror PM2 inconjunction with the laser beam source provided by the third laserscanning station VST1 in the illustrative bioptical laser scanningsystem. The facets of the second polygonal mirror PM2 can be partitionedinto two classes: a first class of facets (corresponding to angles β₁and β₂) have High Elevation (HE) angle characteristics, and a secondclass of facets (corresponding to angles β₃ and β₄) have Low Elevation(LE) angle characteristics; high and low elevation angle characteristicsare referenced by the plane P1 that contains the incoming laser beam andis normal to the rotational axis of the second polygonal mirror PM2;each facet in the first class of facets (having high beam elevationangle characteristics) produces an outgoing laser beam that is directedabove the plane P1 as the facet sweeps across the point of incidence ofthe third laser scanning station VST1; whereas each facet in the secondclass of facets (having low beam elevation angle characteristics)produces an outgoing laser beam that is directed below the plane P1 asthe facet sweeps across the point of incidence of the third laserscanning station VST1.

FIG. 20 depicts the offset between the pre-specified direction ofincidence of the laser beams produced by the laser beam productionmodules of the laser scanning stations HST1 and HST2 and the rotationalaxis of the polygonal mirror PM1. Such offset provides for spatialoverlap in the scanning pattern of light beams produced from thepolygonal mirror PM1 by these laser beam production modules; suchspatial overlap can be exploited such that the overlapping rays areincident on at least one common mirror (mh5 in the illustrativebioptical laser scanning system described herein) to provide a densescanning pattern projecting there from; in the illustrative embodiment,a dense pattern of horizontal planes (groups GH4) is projected from thefront side of the bottom window as is graphically depicted in FIGS. 3F1,3F2 and 4B1 and 4B2.

FIG. 3A illustrates the intersection of the four groups of laserscanning planes (with 20 total scanning planes in the four groups)produced by the first laser scanning station HST1 on the bottom-scanningwindow 16 in the illustrative bioptical laser scanning system describedherein.

FIGS. 3B1 and 3B2 graphically depict a vector-based nomenclature thatmay be used to define horizontal and vertical scanning planes,respectively, that project through the bottom-scanning window 16.

FIGS. 3C1 and 3C2 is a perspective view and top view, respectively, of awire frame model that illustrates the first group GH1 of laser beamfolding mirrors of the first laser scanning station (HST1), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate four different vertical laser scanningplanes that project from the right back corner of the bottom-scanningwindow 16 diagonally outward and upward above the front left side (andfront left corner) of the bottom-scanning window 16 as shown.

FIGS. 3D1 and 3D2 is a front view and top view, respectively, of a wireframe model that illustrates the second group GH2 of laser beam foldingmirrors of the first laser scanning station (HST1), which cooperate withthe four scanning facets of the first rotating polygonal mirror PM1 soas to generate four different horizontal laser scanning planes thatproject from the right side of the bottom-scanning window 16 diagonallyoutward and upward above the left side of the bottom-scanning window 16as shown.

FIGS. 3E1 and 3E2 is a perspective view and top view, respectively of awire frame model that illustrates the third group GH3 of laser beamfolding mirrors of the first laser scanning station (HST1), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate four different vertical laser scanningplanes that project from the right front corner of the bottom-scanningwindow 16 diagonally outward and upward above the back left side andback left corner of the bottom-scanning window 16 as shown.

FIGS. 3F1 and 3F2 is a front view and side view, respectively, of a wireframe model that illustrates the fourth group GH4 of laser beam foldingmirrors of the first laser scanning station (HST1), which cooperate withthe four scanning facets of the first rotating polygonal mirror PM1 soas to generate eight different horizontal laser scanning planes thatproject from the front side of the bottom-scanning window 16 diagonallyoutward and upward above the back side of the bottom-scanning window 16as shown; note that the first laser scanning station HST1 utilizesmirrors MH4 and MH5 (and not MH6) of group GH4 to produce eightdifferent scan planes there from.

FIG. 4A illustrates the intersection of the four groups of laserscanning planes (with 20 total scanning planes in the four groups)produced by the second laser scanning station HST2 on thebottom-scanning window 16 of the illustrative bioptical laser scanningsystem described herein.

FIGS. 4B1 and 4B2 is a front view and side view, respectively, of a wireframe model that illustrates the first group (GH4) of laser beam foldingmirrors of the second laser scanning station (HST2), which cooperatewith the four scanning facets of the first rotating polygonal mirror PM1so as to generate eight different horizontal laser scanning planes thatproject from the front side of the bottom-scanning window 16 diagonallyoutward and upward above the back side of the bottom-scanning window 16as shown; note that the second laser scanning station HST2 utilizesmirrors MH5 and MH6 (and not MH4) of group GH4 to produce eightdifferent scan planes there from.

FIGS. 4C1 and 4C2 is a perspective view and top view, respectively, of awire frame model that illustrates the second group (GH5) of laser beamfolding mirrors of the second laser scanning station (HST2), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate four different vertical laser scanningplanes that project from the left front corner of the bottom-scanningwindow 16 diagonally outward and upward above the back right side andback right corner of the bottom-scanning window 16 as shown.

FIGS. 4D1 and 4D2 is a front view and top view, respectively, of a wireframe model that illustrates the third group (GH6) of laser beam foldingmirrors of the second laser scanning station (HST2), which cooperatewith the four scanning facets of the first rotating polygonal mirror PM1so as to generate four different horizontal laser scanning planes thatproject from the left side of the bottom-scanning window 16 diagonallyoutward and upward above the right side of the bottom-scanning window 16as shown.

FIGS. 4E1 and 4E2 is a perspective view and top-view, respectively, of awire frame model that illustrates the fourth group (GH7) of laser beamfolding mirrors of the second laser scanning station (HST2), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate four different vertical laser scanningplanes that project from the left back corner of the bottom-scanningwindow 16 diagonally outward and upward above the front right side andfront right corner of the bottom-scanning window 16 as shown.

FIG. 4F illustrates the vertical scanning planes that project from thebottom-scanning window 16; including 4 groups (namely, GH1, GH3, GHS andGH7); groups GH1 and GH5 project from opposing portions (e.g., theback-right and front-left corners of the window 16) of thebottom-scanning window 16, and groups GH3 and GH7 project from opposingportions (e.g., front-right and back-left corners of the window 16) ofthe bottom-scanning window; note that groups GHI and GH5 aresubstantially co-planar (i.e., quasi co-planar) and groups GH3 and GH7are substantially co-planar (i.e., quasi co-planar), while groups GHIand GH5 are substantially orthogonal (i.e., quasi-orthogonal) to groupsGH3 and GH7, respectively, as shown.

FIG. 5A illustrates the intersection of the fourteen groups of laserscanning planes (with 28 total scanning planes in the fourteen groups)produced by the third laser scanning station VST1 on the side-scanningwindow 18 of the illustrative bioptical laser scanning system describedherein.

FIGS. 5B1 and 5B2 graphically depict a vector-based nomenclature thatmay be used to define horizontal and vertical scanning planes,respectively, that project through the side-scanning window 18.

FIGS. 5C1 and 5C2 is a front view and top view, respectively, of a wireframe model that illustrates the first group (GV1) of laser beam foldingmirrors of the third laser scanning station (VST1), which cooperate withthe two low-elevation (LE) scanning facets of the second rotatingpolygonal mirror PM2 (corresponding to angles β₃ and β₄ of the secondpolygonal mirror PM2 in FIG. 2N 1) so as to generate two differentvertical laser scanning planes that project from the left side of theside-scanning window 18 diagonally down and out across thebottom-scanning window 16 above the front right corner of thebottom-scanning window 16 as shown.

FIGS. 5D1 and 5D2 is a perspective view and side view, respectively, ofa wire frame model that illustrates the second group (GV2) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two low-elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₃ and β₄ of thesecond polygonal mirror PM2 in FIG. 2N 1) so as to generate twodifferent vertical laser scanning planes that project from the top leftcorner of the side-scanning window 18 downward toward thebottom-scanning window 16 substantially along the left side of thebottom-scanning window 16 as shown.

FIGS. 5E1 and 5E2 is a front view and side view, respectively, of a wireframe model that illustrates the third group (GV3) of laser beam foldingmirrors of the third laser scanning station (VST1), which cooperate withthe two low-elevation scanning facets of the second rotating polygonalmirror PM2 (corresponding to angles β₃ and β₄ of the second polygonalmirror PM2 in FIG. 2N 1) so as to generate two different horizontallaser scanning planes that project from the top left quadrant of theside-scanning window 18 diagonally down across the bottom-scanningwindow 16 as shown.

FIGS. 5F1 and 5F2 is a front view and side view, respectively, of a wireframe model that illustrates the fourth group (GV4) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two low elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₃ and β₄ of thesecond polygonal mirror PM2 in FIG. 2N 1) so as to generate twodifferent horizontal laser scanning planes that project from the topright quadrant of the side-scanning window 18 diagonally down across thebottom-scanning window 16 as shown.

FIGS. 5G1 and 5G2 is a front view and side view, respectively, of a wireframe model that illustrates the fifth group (GV5) of laser beam foldingmirrors of the third laser scanning station (VST1), which cooperate withthe two low-elevation scanning facets of the second rotating polygonalmirror PM2 (corresponding to angles β₃ and β₄ of the second polygonalmirror PM2 in FIG. 2N 1) so as to generate two different vertical laserscanning planes that project from the top right corner of theside-scanning window 18 downward toward the bottom-scanning window 16substantially along the right side of the bottom-scanning window 16 asshown.

FIGS. 5H1 and 5H2 is a front view and side view, respectively, of a wireframe model that illustrates the sixth group (GV6) of laser beam foldingmirrors of the third laser scanning station (VST1), which cooperate withthe two low elevation scanning facets of the second rotating polygonalmirror PM2 (corresponding to angles β₃ and β₄ of the second polygonalmirror PM2 in FIG. 2N 1) so as to generate two different vertical laserscanning planes that project from the right side of the side-scanningwindow 18 diagonally out across the bottom-scanning window 16 above thefront left corner of the bottom-scanning window 16 as shown.

FIGS. 5I1 and 5I2 is a front view and side view, respectively, of a wireframe model that illustrates the seventh group (GV7) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2N 1) so as to generate twodifferent horizontal laser scanning planes that project from the topleft quadrant of the side-scanning window 18 outwardly across thebottom-scanning window 16 (substantially parallel to the bottom-scanningwindow 16) as shown.

FIGS. 5J1 and 5J2 is a front view and top view, respectively, of a wireframe model that illustrates the eighth group (GV8) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2N 1) so as to generate twodifferent horizontal laser scanning planes that project from the leftside of the side-scanning window 18 outwardly across the bottom-scanningwindow 16 (substantially parallel to the bottom-scanning window 16) asshown; in the illustrative embodiment, the characteristic direction ofpropagation of such scanning planes has a non-vertical component whoseorientation relative to the normal of the side-scanning window 18 isgreater than 35 degrees.

FIGS. 5K1 and 5K2 is a front view and side view, respectively, of a wireframe model that illustrates the ninth group (GV9) of laser beam foldingmirrors of the third laser scanning station (VST1), which cooperate withthe two high elevation scanning facets of the second rotating polygonalmirror PM2 (corresponding to angles β₁ and β₂ of the second polygonalmirror PM2 in FIG. 2N 1) so as to generate two different horizontallaser scanning planes that project from the central portion of theside-scanning window 18 outwardly and downward across thebottom-scanning window 16 as shown.

FIGS. 5L1 and 5L2 is a front view and side view, respectively, of a wireframe model, that illustrates the tenth group (GV10) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2N 1) so as to generate twodifferent horizontal laser scanning planes that project from the centralportion of the side-scanning window 18 outwardly and sharply downwardacross the bottom-scanning window 16 as shown.

FIGS. 5M1 and 5M2 is a front view and side view, respectively, of a wireframe model that illustrates the eleventh group (GV11) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2N 1) so as to generate twodifferent horizontal laser scanning planes that project from the centralportion of the side-scanning window 18 outwardly and sharply downwardacross the bottom-scanning window 16 as shown.

FIGS. 5N1 and 5N2 is a front view and side view, respectively, of a wireframe model that illustrates the twelfth group (GV12) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2N 1) so as to generate twodifferent horizontal laser scanning planes that project from the centralportion of the side-scanning window 18 outwardly across thebottom-scanning window 16 (substantially parallel to the bottom-scanningwindow 16) as shown.

FIGS. 5O1 and 5O2 is a front view and top view, respectively, of a wireframe model that illustrates the thirteenth group (GV13) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2N 1) so as to generate twodifferent horizontal laser scanning planes that project from the rightside of the side-scanning window 18 outwardly across the bottom-scanningwindow 16 (substantially parallel to the bottom-scanning window 16) asshown; in the illustrative embodiment, the characteristic direction ofpropagation of such scanning planes has a non-vertical component whoseorientation relative to the normal of the side-scanning window 18 isgreater than 35 degrees.

FIGS. 5P1 and 5P2 is a front view and side view, respectively, of a wireframe model that illustrates the fourteenth group (GV14) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2N 1) so as to generate twodifferent horizontal laser scanning planes that project from the topright quadrant of the side-scanning window 18 outwardly across thebottom-scanning window 16 (substantially parallel to the bottom-scanningwindow 16) as shown.

FIG. 6A is an exemplary timing scheme for controlling the illustrativebioptical laser scanner to cyclically generate a complex omnidirectionalTDM laser scanning pattern from both the bottom and side-scanningwindows 16 and 18 thereof during the revolutions of the scanningpolygonal mirrors PM1 and PM2; in this exemplary timing scheme, foursets of scan plane groups (4*[GH1 . . . GH7]) are produced by stationsHST1 and HST2 during each revolution of the polygonal mirror PM1; twosets of scan plane groups (2*[GV1 . . . GV14]) are produced by stationVST1 during a single revolution of the polygonal mirror PM2; thiscomplex omnidirectional TDM scanning pattern is produced by TDM laserscanning operations (and corresponding signal processing operations) ofthe scanning planes produced by HST1 and HST2 that are defined by aplurality of time slots uniquely assigned to HST1 and HST2,respectively, over each scanning plane produced therefrom. In otherwords, the scan period of a given scanning plane includes a plurality oftime slots logically assigned to the laser scanning station (HST1 orHST2) that produces the given scanning plane.

FIG. 6B illustrates an alternate timing scheme for controlling thebioptical laser scanner of the illustrative embodiment to cyclicallygenerate a complex omni-directional laser scanning pattern from both thebottom and side-scanning windows 16 and 18 thereof during therevolutions of the scanning polygonal mirrors PM1 and PM2; as is similarto the timing scheme of FIG. 6A, four sets of scan plane groups (4* [GH1. . . GH7]) are produced by stations HST1 and HST2 during eachrevolution of the polygonal mirror PM1, and two sets of TDM scan planegroups (2*[GV1 . . . GV14]) are produced by station VST1 during a singlerevolution of the polygonal mirror PM2; in contrast, the TDM laserscanning operations (and corresponding signal processing operations) ofthe scanning planes produced by HST1 and HST2 are defined by time slotsuniquely assigned to HST1 and HST2, respectively, that correspond toscanning planes produced by HST1 and HST2. In other words, the scanperiod of a given scanning plane corresponds to a time slot logicallyassigned to the laser scanning station (HST1 or HST2) that produces thegiven scanning plane.

FIGS. 6C1 through 6C3 illustrates alternate timing schemes forcontrolling the bioptical laser scanner of the illustrative embodimentto cyclically generate a complex omni-directional TDM laser scanningpattern from both the bottom and side-scanning windows 16 and 18 thereofduring the revolutions of the scanning polygonal mirrors PM1 and PM2; asis similar to the timing schemes of FIGS. 6A and 6B, four sets of scanplane groups (4* [GH1 . . . GH7]) are produced by stations HST1 and HST2during each revolution of the polygonal mirror PM1, and two sets of TDMscan plane groups (2*[GV1 . . . GV14]) are produced by station VST1during a single revolution of the polygonal mirror PM2; in contrast, theTDM laser scanning operations (and corresponding signal processingoperations) are defined by time slots uniquely assigned to scanningstations HST1 and HST2 which correspond to scanning plane groupsproduced by stations HST1 and HST2, respectively. In FIG. 6B 1, the TDMtimeslots are logically assigned to scanning plane groups produced byHST1 and HST2 during a_revolution of the polygonal mirror PM1. In FIG.6B 2, the TDM timeslots are logically assigned to scanning plane groupsproduced by HST1 and HST2 during a_revolution of the polygonal mirrorPM1. And in FIG. 6B 3, the TDM timeslots are logically assigned toscanning plane groups produced by HST1 and HST2 during a full revolutionof the polygonal mirror PM1.

FIG. 7 is a functional block diagram of an illustrative embodiment ofthe electrical subsystem of the illustrative bioptical laser scanningsystem in accordance with the present invention, including:photodetectors (e.g. a silicon photocell) for detection of optical scandata signals generated by the respective laser scanning stations; signalconditioning circuitry for conditioning (e.g., preamplification and/orfiltering out unwanted noise in) the electrical signals output by thephotodetectors; bar code symbol detection circuitry that forms adigitized representation (e.g., a sequence of binary bit values) of abar code label being read from signals derived from the output of thesignal conditioning circuitry; bar code digitization circuitry thatconverts the digitized representation of the bar code symbol being readinto a corresponding digital word value; bar code symbol decodecircuitry that decodes the digital word value of the bar code symbolbeing read to generate character data string values associatedtherewith; a programmed microprocessor with a system bus and associatedprogram and data storage memory, for controlling the system operation ofthe bioptical laser scanner and performing other auxiliary functions andfor receiving bar code symbol character data (provided by the bar codesymbol decoding circuitry); a data transmission subsystem forinterfacing with and transmitting symbol character data and otherinformation to host computer system (e.g. central computer, cashregister, etc.) over a communication link therebetween; and aninput/output interface for providing drive signals to anaudio-transducer and/or LED-based visual indicators used to signalsuccessful symbol reading operations to users and the like, forproviding user input via interaction with a keypad, and for interfacingwith a plurality of accessory devices (such as an external handheldscanner, a display device, a weigh scale, a magnetic card reader and/ora coupon printer as shown). In addition, the system includes VLD controlcircuitry that operates in response to the laser control signalssupplied by the microprocessor to generate timing signals HST1_ENABLE,HST2_ENABLE and VST1_ENABLE that define time slots that are logicallyassigned to unique VLD modules (HST1 VLD, HST2 VLD or VST1 VLD) andcorresponding photosensors (PD_(HST1), PD_(HST2), or PD_(VST1)).Modulation enabled VLD drive circuitry operates in response to thetiming signals generated by the VLD control circuitry to modulate theoutput optical power of the three VLD devices (HST1 VLD, HST2 VLD andVST1 VLD) such that, during each given time slot, the VLD logicallyassigned thereto is turned ON and projected into the scanning volumewhile the other VLDs (those not logically assigned to the given timeslot) are turned substantially OFF. In addition, the signal processingcircuitry that performs signal conditioning and/or bar code detectionand operations on the data signals derived from the three photosensors(PD_(HST1), PD_(HST2), and PD_(VST1)) operates synchronously in responseto the timing signals generated by the VLD control circuitry such that,during each given time slot, the photosensor logically assigned theretois operably coupled to the signal processing circuitry that performs barcode detection operations on the data signals derived therefrom whilethe other photosensors (those not logically assigned to the given timeslot) are operably decoupled (e.g., electrically isolated) from suchsignal processing circuitry. The frequency of suchtime-division-multiplexed laser scanning operations and correspondingsignal processing operations may be greater than two times the Nyquistfrequency (and preferably performed at a frequency greater than fourtimes the Nyquist frequency). In such embodiments, when a givenphotosensor is operably decoupled (e.g., electrically isolated) from barcode detection signal processing circuitry, such bar code detectionsignal processing circuitry operates on a sample of the data signalsderived from the given photosensor during the previous time slotlogically assigned to the given photosensor. Preferably, this sample isstored on a hold capacitor that is operably coupled to such bar codedetection signal processing circuitry.

FIG. 8A is a functional block diagram of exemplary electronic circuitrythat may be used to derive the timing signals that synchronize thetime-division multiplexed laser scanning and signal processingoperations performed by a laser scanning system in accordance with thepresent invention; for example, the electronic circuitry of FIG. 8A maybe integrated as part of the VLD control module of the illustrativebioptical laser scanning system as shown in FIG. 7.

FIGS. 8B1 through 8B9 illustrate the timing signals generated by thecircuit elements of FIG. 8A; the timing signals define time slots duringwhich time-division multiplexed laser scanning and signal processingoperations are performed in accordance with the present invention.

FIG. 8C is a table that describes time-division multiplexed laserscanning and signal processing operations performed by the illustrativebioptical laser scanning system described herein in accordance with thepresent invention; such time-division multiplexed laser scanning andsignal processing operations are performed during time slots defined bythe timing signals of FIGS. 8B1 through 8B9.

FIG. 8D is a high-level functional block diagram of electronic circuitrythat performs time-division multiplexed laser scanning operations inresponse to the timing signals of FIGS. 8B6 through 8B9.

FIG. 8E 1 is a functional block diagram of exemplary electroniccircuitry that performs time-division multiplexed laser scanningoperations in response to the timing signals of FIGS. 8B6 through 8B9.

FIG. 8E 2 is a table that describes VLD modulation operations performedby the electronic circuitry of FIG. 8E 1 for one laser light source(e.g., the VLD of laser scanning station HST1) in response to the timingsignal HST1_ENABLE of FIG. 8B 6; analogous VLD modulation operation areperformed by the electronic circuitry of FIG. 8E 1 for other laser lightsources (e.g., the VLDs of laser scanning stations HST2 and VST1) inresponse to the timing signals HST2_ENABLE and VST1_ENABLE of FIGS. 8B7and 8B8, respectively, to perform the time-division multiplexed laserscanning operations in accordance with the present invention.

FIG. 8E 3 is a graphical representation of the optical powercharacteristics output from an exemplary visible laser diode in responseto current supplied thereto; in addition the graphical representationshows the output power levels P_(ON) and P_(OFF) of the exemplaryvisible laser diode in response to the logic levels H and L,respectively, of the enable timing signal (e.g., HST1_ENABLE) during thetime-division multiplexed laser scanning operations of FIG. 8E 2.

FIG. 8F 1 is a functional block diagram of an illustrative embodiment ofHST1 VLD Drive Circuitry and HST1 VLD Module in accordance with thepresent invention, such circuitry carries out time-division multiplexedlaser scanning operations for HST1 in response to the timing signals ofFIGS. 8B6 through 8B8, in accordance with the table of FIG. 8E 2 andgraph of FIG. 8E3; analogous circuitry may be embodied as part of theHST2 Drive Circuitry and HST2 VLD Module as well as the VST1 DriveCircuitry and VST1 VLD Module, which carries out time-divisionmultiplexed laser scanning operations for HST2 and VST1, respectively,in response to the timing signals of FIGS. 8E6 through 8E8.

FIG. 8F 2 is a graphical representation of the optical powercharacteristics output from an exemplary visible laser diode (which ispart of the HST1 VLD Module, HST2 VLD Module, or VST1 Module shown inFIG. 8F 1) in response to current supplied thereto; in addition, thegraph shows the output power levels P_(ON) and P_(OFF) of the visiblelaser diode in response to the logic levels H and L, respectively, ofthe corresponding enable timing signal (e.g., HST1_ENABLE, HST2_ENABLE,or VST1_ENABLE) during time-division multiplexed laser scanningoperations; note that in the illustrative embodiment of FIGS. 8F1 and8F2, the current level I_(Laser) that is supplied to the visible laserdiode is near zero (I_(Laser)=I_(MOD)=I_(Off)≈0) in response to the “L”logic level of the enable timing signal in order to produce the P_(OFF)output power level (e.g., near zero output power level), and that thecurrent level I_(Laser) that is supplied to the visible laser diode issubstantially greater than the threshold current I_(TH)(I_(Laser)=I_(MOD)=I_(ON)>>I_(TH)) in response to the “H” logic level ofthe enable timing signal in order to produce the P_(ON) output powerlevel (which is substantially greater than near zero, e.g., on the orderof 1 to 5 mW for typical bar code reading applications).

FIG. 8G 1 is a functional block diagram of an alternate embodiment ofHST1 VLD Drive Circuitry and HST1 VLD Module in accordance with thepresent invention, such circuitry carries out time-division multiplexedlaser scanning operations for HST1 in response to the timing signals ofFIGS. 8B through 8B8, in accordance with the table of FIG. 8E 2 andgraph of FIG. 8E 3; analogous circuitry may be embodied as part of theHST2 Drive Circuitry and HST2 VLD Module as well as the VST1 DriveCircuitry and VST1 VLD Module, which carries out time-divisionmultiplexed laser scanning operations for HST2 and VST1, respectively,in response to the timing signals of FIGS. 8B6 through 8B8.

FIG. 8G 2 is a graphical representation of the optical powercharacteristics output from an exemplary visible laser diode (which ispart of the HST1 VLD Module, HST2 VLD Module, or VST1 Module shown inFIG. 8G 1) in response to current supplied thereto; in addition, thegraph shows the output power levels P_(ON) and P_(OFF) of the visiblelaser diode in response to the logic levels H and L, respectively, ofthe corresponding enable timing signal (e.g., HST1_ENABLE, HST2_ENABLE,or VST1_ENABLE) during time-division multiplexed laser scanningoperations; note that in the illustrative embodiment of FIG. 8G 1 and8G2, the current level I_(Laser) that is supplied to the visible laserdiode is near the threshold current I_(TH)(I_(Laser)=I_(BIAS)=I_(Off)≈I_(TH)) in response to the “L” logic levelof the enable timing signal in order to produce the P_(OFF) output powerlevel (e.g., near zero output power level), and that the current levelI_(Laser) that is supplied to the visible laser diode is substantiallygreater than the threshold current I_(TH)(I_(Laser)=(I_(BIAS)+I_(MOD))=I_(ON)>>I_(TH)) in response to the “H”logic level of the enable timing signal in order to produce the P_(ON)output power level (which is substantially greater than near zero, e.g.,on the order of 1 to 5 mW for typical bar code reading applications).

FIG. 9A is a functional block diagram of exemplary electronic circuitrythat performs time-division multiplexed signal processing operations inresponse to the timing signals of FIGS. 8B6 through 8B8 for theillustrative bioptical laser scanning system whose system architectureis shown in FIG. 7; the frequency of such time-division-multiplexedlaser scanning operations and corresponding signal processing operationsmay be greater than two times the Nyquist frequency (and preferablyperformed at a frequency greater than four times the Nyquist frequency);in this illustrative embodiment, track and hold circuitry operates inresponse to the timing signals of FIGS. 8B6 through 8B8 to operablycouple/de-couple the photosensors PD_(HST1), PD_(HST2) and PD_(VST1)from signal processing circuitry that performs bar code detection anddecoding on data signals derived from the output of the photosensorsPD_(HST1), PD_(HST2) and PD_(VST1) during the time slot assigned tophotosensor PD_(HST1), PD_(HST2), PD_(VST1) (and laser scanning stationHST1, HST2 or VST1), respectively; in such embodiments, during the“hold” mode when a given photosensor is operably decoupled (e.g.,electrically isolated) from bar code detection signal processingcircuitry, such bar code detection signal processing circuitry operateson a sample of the data signals derived from the given photosensorduring the previous time slot logically assigned to the givenphotosensor. Preferably, this sample is stored on a hold capacitor thatis operably coupled to such bar code detection signal processingcircuitry.

FIG. 9B is a circuit diagram of an exemplary track and hold circuitsuitable for use in the time-division multiplexed signal processingcircuitry of FIG. 9A; the track and hold circuit includes an inputbuffer (provided by a 0.01 uF capacitor and 5 Kohm resistor for ACcoupling, an op-amp configured as a voltage follower, and 10 K ohmresistor coupled between the output node of the op-amp and ground), aFET switch whose source/drain is coupled between the output node of theinput buffer and input node of an output buffer, and whose gate iscoupled to a signal line that carries the SourceX_Enable signal (toswitch between “track mode” and “hold mode”), a 470 pF hold capacitorcoupled between the input node of the output buffer and ground, and anoutput buffer (provided an op-amp configured as a non-invertingamplifier with a gain of 3, e.g. 1+300/150).

FIG. 9C is a functional block diagram of exemplary electronic circuitrythat performs time-division multiplexed signal processing operations inresponse to the timing signals of FIGS. 8B6 through 8B9; in thisillustrative embodiment, three analog to digital signal converters areprovided, each processing data signals derived from a differentphotosensor (PD_(HST1), PD_(HST2) or PD_(VST1)). The three analog todigital converters each sample the conditioned analog scan data signalsat a sampling frequency at least two times the highest frequencycomponent expected in the analog scan data signal (in accordance withthe well known Nyquist criteria) and quantizes each time-sampled scandata signal value into a discrete signal level using a suitable lengthnumber representation (e.g. 8 bits) to produce a discrete scan datasignal. One or more programmed processors (e.g., a digital signalprocessor and associated memory) perform digital signal processingfunctions on the discrete signal levels to generate a sequence ofdigital words (i.e., a sequence of digital count values), eachrepresenting the time length associated with the signal leveltransitions in the corresponding digital scan data signal as describedabove; time-division multiplexed signal processing operations areprovided by selectively enabling the sampling function (and possibly theconversion function) of only one of the three analog to digital signalconverters during the time slot assigned to photosensor PD_(HST1),PD_(HST2) or PD_(VST1) (and laser scanning station HST1, HST2 or VST1)corresponding thereto.

FIG. 9D is a functional block diagram of exemplary electronic circuitrythat performs time-division multiplexed signal processing operations inresponse to the timing signals of FIGS. 18A6 through 18A8; in thisillustrative embodiment, a shared analog to digital signal converter isprovided that processes data signals derived from any one of the threephotosensors PD_(HST1), PD_(HST2) and PD_(VST1). Multiplexing switchesoperate in response to the timing signals of FIGS. 8B6 through 8B9 tooperably couple/de-couple a given photosensor (PD_(HST1), PD_(HST2) orPD_(VST1)) to the shared analog to digital signal converter during eachtime slot assigned to the given photosensor (PD_(HST1), PD_(HST2) orPD_(VST1) (and laser scanning station HST1, HST2 or VST1). The sharedanalog to digital converter samples the conditioned analog scan datasignals at a sampling frequency at least two times the highest frequencycomponent expected in the analog scan data signal (in accordance withthe well known Nyquist criteria) and quantizes each time-sampled scandata signal value into a discrete signal level using a suitable lengthnumber representation (e.g. 8 bits) to produce a discrete scan datasignal. One or more programmed processors (e.g., a digital signalprocessor and associated memory) perform digital signal processingfunctions on the discrete signal levels to generate a sequence ofdigital words (i.e., a sequence of digital count values), eachrepresenting the time length associated with the signal leveltransitions in the corresponding digital scan data signal as describedabove; time-division multiplexed signal processing operations areprovided by selectively enabling the sampling function (and possibly theconversion function) of the shared analog to digital signal converterduring the time slots assigned to the photosensors PD_(HST1), PD_(HST2)and PD_(VST1) (and laser scanning station HST1, HST2 and VST1)corresponding thereto.

FIG. 10 is a schematic representation of an alternate embodiment of thepresent invention wherein a time division multiplexed beam steeringdevice provides time-division multiplexed laser scanning beams to thethree laser scanning stations HST1, HST2 and VST1 over time intervalsT1, T2 and T3 assigned thereto.

FIGS. 11A and 11B are graphical representations of the power spectrum ofan exemplary analog scan data signal produced when laser scanning a barcode symbol within near and far focal zones of a laser scanning system,shown plotted along with the power density spectrum of thepaper/substrate noise signal produced while laser scanning the bar codesymbol on its substrate within such near and far focal zones.

FIG. 11C is a functional block diagram of an illustrative embodiment ofa TDM-enabled multi-path scan data signal processor according to thepresent invention, including: signal conditioning circuitry 903, trackand hold circuitry 911, and low pass filter circuitry 912 operablycoupled between a photodetector 902 and a plurality of signal processingpaths (two shown as path A and path B); The signal conditioningcircuitry 903 operates to amplify (and possible filter out unwantednoise components in) the scan data signal produced by the photodetector902; The track and hold circuitry 911 operates in response to timingsignal SourceX-Enable, which defines time slots logically assigned tothe photodetector 902, to operably couple/decouple the output of thesignal conditioning circuitry 903 to the low pass filter 912 in responseto the timing signal SourceX-Enable; when the SourceX-Enable signal isactive (“track” mode), the output of the signal conditioning circuitry903 is operably coupled to the low pass filter 912; when theSourceX-Enable signal is inactive (“hold” mode), the output of thesignal conditioning circuitry 903 is operably decoupled (e.g.,electrically isolated) from the low pass filter 912; in addition, in the“hold” mode, the hold capacitor of the track and hold circuit 911 storesa sample of the data signals derived from the photosensor 902 during theprevious time slot assigned to photosensor 902, and the voltage level atthe output of track and hold circuit 911 corresponds to the sampledvoltage level stored on the hold capacitor; the low pass filter 912operates to smooth out or otherwise filter the data signal supplied thetrack and hold circuit 911; the output of the low pass filter 912 isprovided to the plurality of signal processing paths for processing inparallel; each signal processing path includes: a first derivativesignal generation circuit 904 having a differentiator, low pass filterand amplifier therein; a second derivative signal generation circuit 906having a differentiator therein; a first derivative signalthreshold-level generation circuit 905; and a zero crossing detector907, data gate 908, and binary-type A/D signal conversion circuitry 909;each signal processing path has different operational characteristics(such as different cutoff frequencies in the filtering stages of thefirst and second derivative signal generation circuits of the respectivepaths, different gain characteristics in amplifier stages of the firstand second derivative signal generation circuits of the respectivepaths, and/or different positive and negative signal thresholds in thefirst derivative threshold circuitry of the respective paths); thevarying operational characteristics of the paths provide differentsignal processing functions.

FIGS. 12A through 12I are signal diagrams that illustrate the operationof the TDM-enabled multi-path scan data signal processor 901 of theillustrative embodiment of FIG. 11C; FIG. 12A depicts the TDM scanninglaser beam moving across bar and spaces of a bar code; FIG. 12B 1depicts the light level received at the photodetector 902; FIG. 12B 2depicts the light level received at the photodetector at high resolutionin time, which shows the effect of TDM laser scanning; FIG. 12C 1depicts the voltage level produced at the output node of the track andhold circuit at a high resolution in time, which shows the effect of TDMsignal processing operations; FIG. 12C 2 depicts the voltage level ofthe scan data signal produced at the output of filter 912; and FIGS. 12Dthrough 12I depict the processing performed in one of the respectivepaths of the multi-path scan data signal processor 901; Similarprocessing operations with different operations characteristics areperformed in other paths of the multi-path scan data signal processor901.

FIGS. 13A and 13B, taken together, illustrate an exemplary embodiment ofthe signal conditioning circuitry 903, track and hold circuit 911 andlow pass filter 912 of FIG. 11C, which performs TDM signal processingoperations that amplify and smooth out or otherwise filter the scan datasignal produced by the photodetector 902 to remove unwanted noisecomponents therein over time slots defined by the SourceX_Enable timingsignal provided thereto.

FIG. 14 is a schematic diagram illustrating an exemplary implementationof the first derivative signal generation circuitry 904, which issuitable for use in the two different paths of the scan data signalprocessor of FIG. 11C, including a number of subcomponents arranged in aserial manner that process the analog scan data signal produced by thesignal conditioning circuitry 903, namely: a differentiator stage 1201,a low-pass filter (LPF) stage 1203, and an amplifier stage 1205.

FIG. 15 is a schematic diagram illustrating an exemplary implementationof the second derivative signal generation circuitry 906, which issuitable for use in the two different paths of the scan data signalprocessor of FIG. 11C, including: a differentiator stage 1301 thatgenerates a signal whose voltage level is proportional to the derivativeof the first derivative signal produced by the first derivativegeneration circuitry 904 (thus proportional to the second derivative ofthe analog scan data signal produced by the signal conditioningcircuitry 903) for frequencies in a predetermined frequency band.

FIGS. 16A through 16D, taken all together, set forth a schematic diagramillustrating an exemplary implementation of the first derivative signalthreshold circuitry 905, which is suitable for use in the two differentpaths of the scan data signal processor of FIG. 11C, including: anamplifier stage 1401 that amplifies the voltage levels of the firstderivative signal produced by the first derivative signal generationcircuitry 904, positive and negative peak detectors 1403 and 1405, and acomparator stage 1407 that generates output signals (e.g., theUpper_Threshold Signal and Lower_Threshold Signal) that indicate thetime period when the positive and negative peaks of the amplified firstderivative signal produced by the amplifier stage exceed predeterminedthresholds (i.e., a positive peak level PPL and a negative peak levelNPL).

FIG. 17 illustrates an exemplary implementation of a zero crossingdetector 907, which is suitable for use in the two different paths ofthe scan data signal processor of FIG. 11C, including a comparatorcircuit that compares the second derivative signal produced from thesecond derivative generation circuit in its respective path with a zerovoltage reference (i.e. the AC ground level) provided by the zeroreference signal generator, in order to detect the occurrence of eachzero-crossing in the second derivative signal, and provide outputsignals (ZC_1 and ZC_2 signals) identifying zero crossings in the secondderivative signal.

FIG. 18 is a schematic diagram illustrating an exemplary implementationof the data gating circuitry 908 and 1-Bit A/D conversion circuitry 909,which is suitable for use in the two different paths of the scan datasignal processor of FIG. 11C.

FIG. 19A is a functional block diagram of a system architecture suitablefor a digital implementation of the TDM-enabled scan data signalprocessor of the present invention.

FIGS. 19B through 19D are functional block diagrams that illustrateexemplary digital implementations of the TDM-enabled multi-path scandata processing according to the present invention, wherein digitalsignal processing operations are preferably carried out on the discretescan data signal levels generated by the A/D converter 1705 and storedin the memory 1709 of FIG. 19A; FIG. 19B illustrates exemplary digitalsignal processing operations that identify a data frame (e.g., a portionof the discrete scan data signal levels stored in memory 1709) thatpotentially represents a bar code symbol (block 1723) and stores thedata frame in a working buffer (block 1725); FIG. 19C illustratesexemplary digital signal processing operations that carry out multi-pathscan data signal processing according to the present invention; and FIG.19D illustrates alternative digital signal processing operations thatcarry out multi-subpath scan data signal processing (with differentfirst derivative threshold processing performed in each subpath)according to the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS OF THE PRESENTINVENTION

Referring to the figures in the accompanying Drawings, the variousillustrative embodiments of the bioptical laser scanner of the presentinvention will be described in great detail.

FIG. 1A is a pictorial illustration of an exemplary bar code symbolscanning system employing time-division multiplexed laser scanning andsignal processing operations in accordance with the present invention.In the illustrative embodiment shown, two time-division multiplexedlaser scanning beams cyclically scan through the scanning volume of thesystem for detecting and decoding bar code labels disposed on surfacestherein. However, the present invention is not limited in this respectand can be employed in bar code systems that cyclically scan through thescanning volume of the system with a larger number of time-divisionmultiplexed laser scanning beams (for example, the illustrativebioptical laser scanning system described below).

More specifically, during each scan cycle through the scanning volume,each laser scanning beam forms a plurality of multi-dimensional scanlines that project through the scanning volume such that there is anoverlap period between scan periods for scan line pairs (or scan linegroups) produced by the two laser scanning beams. FIG. 1B is a timingdiagram that illustrates the overlap period between scan periods for tworepresentative scan lines (or scan line groups) produced by the twolaser scanning beams of FIG. 1A. In accordance with the presentinvention, time-division multiplexed laser scanning operations andsignal processing operations are performed during this overlap periodwhereby a plurality of successive non-overlapping time slots (that occurduring this overlap period) are logically assigned to a unique laserscanning beam and corresponding photosensor. During a given time slot,the laser scanning beam logically assigned thereto (e.g., Laser ScanningBeam 1) is selectively generated (or selectively projected) into thescanning volume while generation (or projection) of the other laserscanning beam (e.g., Laser Scanning Beam 2) is disabled. Moreover,during the given time slot (or during a portion of the given time slot),the photosensor logically assigned thereto (e.g., PD1) is operablycoupled to signal processing circuitry that performs bar code detectionoperations on the data signals derived therefrom. Furthermore, duringthe given time slot, the other photosensor (e.g., PD2) is operablydecoupled (e.g., electrically isolated) from bar code detection signalprocessing circuitry that performs bar code detection operations on thedata signals derived therefrom.

Such time-division-multiplexed laser scanning operations andcorresponding signal processing operations may be performed at highfrequencies (for example, frequencies greater than two times the Nyquistfrequency, and possibly at frequencies greater than four times theNyquist frequency as described herein). In such embodiments, when agiven photosensor is operably decoupled (e.g., electrically isolated)from bar code detection signal processing circuitry, such bar codedetection signal processing circuitry operates on a sample of the datasignals derived from the given photosensor during the previous time slotlogically assigned to the given photosensor. Such operations arepreferably provided by track and hold circuitry (or sample and holdcircuitry which is well known in the electronic arts) which operablycouples/decouples (e.g., electrically isolates) the output of a givenphotosensor from bar code detection signal processing circuitry, andstores a sample of the data signals derived from the given photosensorduring the previous time slot logically assigned to the givenphotosensor on a hold capacitor that is operably coupled to bar codedetection signal processing circuitry.

Alternatively, such time-division-multiplexed laser scanning operationsand corresponding signal processing operations may be performed at lowerfrequencies (e.g., much less than the Nyquist frequency, such as atfrequencies corresponding to scan lines or scan line groups as describedherein). In such embodiments, a multiplexing switch may be used tooperably couple/decouple (e.g., electrically isolate) the output of agiven photosensor from bar code detection signal processing circuitry,and there is no need to store (and process) a sample of the data signalsderived from the given photosensor during the previous time slotlogically assigned to the given photosensor. It should be noted that insuch embodiments, the bar code detection signal processing circuitryoperates at frequencies at least two times greater than the Nyquistfrequency in sampling and processing the scan data signals suppliedthereto over a given TDM time slot to detect and decode bar code symbolstherein.

FIGS. 1C1 and 1C2 illustrate timing signals that define time slotsduring the overlap period of FIG. 1B whereby time-division multiplexedlaser scanning and signal processing operations are performed (as setforth in the table of FIG. 1D) during such time slots in accordance withthe present invention. As shown, the active state (e.g., H logic level)of the SOURCE1_ENABLE signal defines a set of time slots (denoted“slot_1”) that are logically assigned to laser source LS1 andcorresponding photosensor (e.g. PD1) of FIG. 1A. Similarly, the activestate (e.g., H logic level) of the SOURCE2_ENABLE signal defines a setof time slots (denoted “slot_2”) that are logically assigned to LaserSource LS2 and corresponding photosensor (e.g. PD2) of FIG. 1A. The“slot_1” time slots do not temporally overlap the “slot_2” time slots asshown.

As summarized in the table of FIG. 1D, the following operations areperformed during the “slot_1” time slots when the Source1_Enable Signalis active:

-   -   i) during the “slot_1” time slots, generation and/or projection        of the scanning laser beam from Laser Source LS1 into the        scanning volume is enabled; and    -   ii) during a portion of the “slot_1” time slots, photosensor PD1        is operably coupled to signal processing circuitry that performs        bar code detection and decoding operations on data signals        derived from the output of photosensor PD1.

Note that during the “slot_1” time slots, the Source2_Enable Signal isinactive. As summarized in the table of FIG. 1D, the followingoperations are performed during the “slot_1” time slots when theSource2_Enable Signal is inactive:

-   -   i) generation and/or projection of the scanning laser beam from        Laser Source LS2 into the scanning volume is disabled; and    -   ii) photosensor PD2 is operably decoupled (e.g., electrically        isolated) from bar code detection signal processing circuitry.

In the event that the time-division-multiplexed laser scanningoperations of Laser Source LS2 and corresponding signal processingoperations performed on the output of photosensor PD2 are performed at ahigh frequency (e.g., at a frequency greater than two times the Nyquistfrequency and possibly greater than four times the Nyquist frequency asdescribed herein), which is determined by the frequency ofSource2_Enable Signal, bar code detection signal processing circuitryoperates on a sample of the data signals derived from the photosensorPD2 during the previous “slot_2” time slot. Such operations arepreferably provided by track and hold circuitry (or sample and holdcircuitry which is well known in the electronic arts) which operablycouples/decouples (e.g., electrically isolates) the output of aphotosensor PD2 from bar code detection signal processing circuitry, andstores a sample of the data signals derived from the photosensor PD2during the previous “slot_2” time slot on a hold capacitor that isoperably coupled to bar code detection signal processing circuitry whenthe Source2_Enable Signal is inactive (which is during the “slot_1” timeslots).

In the event that the time-division-multiplexed laser scanningoperations of Laser Source LS2 and corresponding signal processingoperations performed on the output of photosensor PD2 are performed at alower frequency (e.g., at a frequency much less than the Nyquistfrequency as described herein), which is determined by the frequency ofSource2_Enable Signal), bar code detection signal processing circuitrydoes not operate on samples of data signals derived from the photosensorPD2 during the “slot_1” time slots. It should be noted that in suchembodiments, the bar code detection signal processing circuitry operatesat frequencies at least two times greater than the Nyquist frequency insampling and processing the data signals derived from the photosensorPD1 and supplied thereto during the “slot_1” time slots in order todetect and decode bar code symbols therein.

Similar time-division multiplexed scanning and signal processingoperations are performed by Laser Source LS2 and the output of PD2during the “slot_2” time slots. More specifically, as summarized in thetable of FIG. 1D, the following operations are performed during the“slot_2” time slots when the Source2_Enable Signal is active:

-   -   i) during the “slot_2” time slots, generation and/or projection        of the scanning laser beam from Laser Source LS2 into the        scanning volume is enabled; and    -   ii) during a portion of the “slot_2” time slots, photosensor PD2        is operably coupled to signal processing circuitry that performs        bar code detection and decoding operations on data signals        derived from the output of photosensor PD2.

Note that during the “slot_2” time slots, the Source1_Enable Signal isinactive. As summarized in the table of FIG. 1D, the followingoperations are performed during the “slot_2” time slots when theSource1_Enable Signal is inactive:

-   -   i) generation and/or projection of the scanning laser beam from        Laser Source LS1 into the scanning volume is disabled; and    -   ii) photosensor PD1 is operably decoupled (e.g., electrically        isolated) from bar code detection signal processing circuitry.

In the event that the time-division-multiplexed laser scanningoperations of Laser Source LS1 and corresponding signal processingoperations performed on the output of photosensor PD1 are performed at ahigh frequency (e.g., at a frequency greater than two times the Nyquistfrequency and possibly greater than four times the Nyquist frequency asdescribed herein), which is determined by the frequency ofSource1_Enable Signal), bar code detection signal processing circuitryoperates on a sample of the data signals derived from the photosensorPD1 during the previous “slot_1” time slot. Such operations arepreferably provided by track and hold circuitry (or sample and holdcircuitry which is well known in the electronic arts) which operablycouples/decouples (e.g., electrically isolates) the output of aphotosensor PD1 from bar code detection signal processing circuitry, andstores a sample of the data signals derived from the photosensor PD1during the previous “slot_1” time slot on a hold capacitor that isoperably coupled to bar code detection signal processing circuitry whenthe Source1_Enable Signal is inactive (which is during the “slot_2” timeslots).

In the event that the time-division-multiplexed laser scanningoperations of Laser Source LS1 and corresponding signal processingoperations performed on the output of photosensor PD1 are performed at alower frequency (e.g., at a frequency much less than the Nyquistfrequency as described herein), which is determined by the frequency ofSource1_Enable Signal, bar code detection signal processing circuitrydoes not operate on samples of data signals derived from the photosensorPD1 during the “slot_2” time slots. It should be noted that in suchembodiments, the bar code detection signal processing circuitry operatesat frequencies at least two times greater than the Nyquist frequency insampling and processing the data signals derived from the photosensorPD2 and supplied thereto during the “slot_2” time slots in order todetect and decode bar code symbols therein.

Advantageously, both the higher frequency and lower frequencysynchronous time-division multiplexed laser scanning and signalprocessing operations as described herein enable a bar code symbolscanning system of the present invention to scan the scanning volumewith a plurality of multi-dimensional laser scanning beams to detect anddecode bar code symbols on surfaces disposed therein while avoidingoptical crosstalk in addition to light interference from ambient lightand unwanted reflections. In addition, the lower frequency synchronoustime-division multiplexed laser scanning and signal processingmechanisms as described herein are less complex and less costly toimplement as compared to the higher frequency mechanisms, yet such lowerfrequency mechanisms may suffer from decreased throughput because thebar code detection signal processing operations is inactive duringportions of the scan cycle.

The synchronous time-division multiplexed laser scanning and signalprocessing operations described herein are well suited for a point ofsale (POS) presentation scanner (where a label to be scanned is movedthrough the scanning region for data acquisition); however such featurescan be used in other bar code reading and imaging systems, includinghandheld scanners and other POS scanners in addition to hold-underscanners and other industrial scanners.

Point-of-sale (POS) scanners are typically designed to be used at aretail establishment to determine the price of an item being purchased.POS scanners are generally smaller than industrial scanner models, withmore artistic and ergonomic case designs. Small size, low weight,resistance to damage from accident drops and user comfort, are all majordesign factors for the POS scanner. POS scanners include hand-heldscanners, hands-free presentation scanners and combination-type scannerssupporting both hands-on and hands-free modes of operation. Thesescanner categories will be described in greater detail below.

As described above, hand-held scanners are designed to be picked up bythe operator and aimed at the label to be scanned. In addition,hand-held scanners have many uses outside POS applications such asinventory management and portable data acquisition and objectidentification.

Hands-free presentation scanners are designed to remain stationary andhave the item to be scanned picked up and passed in front of thescanning device. Presentation scanners can be mounted on counterslooking horizontally, embedded flush with the counter lookingvertically, or partially embedded in the counter looking vertically, buthaving a “tower” portion which rises out above the counter and lookshorizontally to accomplish multiple-sided scanning. If necessary,presentation scanners that are mounted in a counter surface can alsoinclude a scale to measure weights of items.

Some POS scanners can be used as handheld units or mounted in stands toserve as presentation scanners, depending on which is more convenientfor the operator based on the item that must be scanned.

An industrial scanner is a scanner that has been designed for use in awarehouse or shipping application where large numbers of packages mustbe scanned in rapid succession. Industrial scanners includeconveyor-type scanners (which scan packages as they move by on aconveyor belt) and hold-under scanners (which scan packages that arepicked up and held underneath it). The package is then manually routedor otherwise handled, perhaps based on the result of the scanningoperation. Hold-under scanners are generally mounted so that its viewingoptics are oriented in a downward direction, like a library bar codescanner.

An illustrative bioptical scanning system (presentation-type) inaccordance with the present invention is illustrated in FIGS. 2A through7. As shown in FIGS. 2A-2C, the housing 5′ of the scanner has multipleparts (a bottom portion 5A′, a top portion 5B′ and a hood portion 5C′)that are preferably mated together with screws and posts as shown. Thetop portion 5B′ includes a first scanning window 16 (referred to belowas the “bottom scanning window”), while the hood portion 5C′ includes asecond scanning window 18 (referred to below as “side scanning window”)which is preferably oriented substantially orthogonal to the bottomscanning window 16 as shown. When the scanning system is installedwithin a counter-top surface, as shown in FIG. 2D, the top portion 5B′(and the bottom scanning window 16 integral thereto is orientedhorizontally, whereas the hood portion 5C′ (and the side scanning window18 integral thereto) is oriented vertically with respect to the POSstation. Thus throughout the Specification and Claims hereof, the terms“bottom scanning window” and “horizontal window” may be usedinterchangeably but refer to the same structure; likewise, the terms“side scanning window” and “vertical window” may be used interchangeablybut refer to the same structure.

The bottom housing portion 5A′ and top housing portion 5B′ together(which include the bottom scanning window 16) have width, length andheight dimensions of approximately 11.405, 14.678 and 3.93 inches,respectively, whereas the hood housing portion 5C′ (which includes theside scanning window 18) has width and height dimensions of 12.558inches and 7.115 inches, respectively. The total height of the scannerhousing 5′ is approximately 11.044 inches. In addition, thebottom-scanning window 16 has width and length dimensions ofapproximately 3.94 inches (100 mm) and 5.9 inches (150 mm),respectively, to provide a window with a square area of approximately15,000 square mm. And, the side-scanning window 18 has width and heightdimensions of approximately 9.8 inches (248 mm) and 5.9 inches (150 mm),respectively, to provide a window with a square area of approximately37,200 square mm. As will be described in greater detail below, thebioptical laser scanning mechanism housed within this housing producesan omnidirectional TDM laser scanning pattern within thethree-dimensional volume above the bottom-scanning window 16 and infront of the side-scanning window 18.

The omnidirectional TDM scanning pattern is capable of readingpicket-fence type bar code symbols on bottom-facing surfaces (i.e., asurface whose normal is directed toward the bottom-scanning window 16 ofthe scanner), top-facing surfaces (i.e., a surface whose “flip-normal”is directed toward the bottom-scanning window 16 of the scanner),back-facing surfaces (i.e., a surface whose normal is directed towardthe side-scanning window 18 of the scanner), front-facing surfaces(i.e., a surface whose “flip-normal” is directed toward theside-scanning window 18 of the scanner), left-facing surfaces (i.e., asurface whose normal is directed toward or above the left side of thescanner), and right-facing surfaces (i.e., a surface whose normal isdirected toward or above the right side of the scanner). A “flip-normal”as used above is a direction co-linear to the normal of a surface yetopposite in direction to this normal as shown in FIG. 2E. An example ofsuch bottom-facing, top-facing, back-facing, front-facing surfaces,left-facing surfaces, and right-facing surfaces of a rectangular shapedarticle oriented in the scan volume of the bioptical laser scanningsystem 1′ disposed between bottom-scanning and side-scanning windows 16,18 of the system is illustrated in FIG. 2D.

The illustrative bioptical laser scanning system 1′ can be used in adiverse variety of bar code symbol scanning applications. For example,the bioptical laser scanner 1′ can be installed within the countertop ofa point-of-sale (POS) station as shown in FIG. 2F. In this application,it is advantageous to integrate a weight scale with the laser scanningmechanism. Such a device is described in detail in U.S. patentapplication Ser. No. 10/045,577 (Attorney Docket No. 108-120USA000),incorporated by reference above in its entirety. As shown in FIG. 2F,the bioptical laser scanner 1′ can be installed within the countertop ofa point-of-sale (POS) station 51, having a computer-based cash register53, a weigh-scale 55 mounted within the counter adjacent the laserscanner 1′ (or integral to the scanner), and an automated transactionterminal (ATM) 57 supported upon a courtesy stand in a conventionalmanner.

As shown in FIGS. 2G through 2M, the illustrative bioptical scanningsystem 1′ includes two sections: a first section (sometimes referred toas the horizontal section) disposed within the bottom housing portion5A′ and top housing portion 5B′ and a second section (sometimes referredto as the vertical section) substantially disposed within the bottomhousing portion 5A′ and the hood housing portion 5C′. It should be notedthat in the illustrative embodiment, parts of the vertical section aredisposed within the back of the bottom housing portion 5A′ as willbecome evident from the figures and accompanying description thatfollows. Also note that horizontal section includes components mountedon the first scan module insert 3A′ as set forth above, while thevertical section includes components mounted on the second scan moduleinsert 3B′ as set forth above.

As shown in FIGS. 2G through 2J (and in tables I and II below), thefirst section includes a first rotating polygonal mirror PM1, and firstand second scanning stations (indicated by HST1 and HST2, respectively)disposed thereabout. The first and second laser scanning stations HST1and HST2 each include a laser beam production module (not shown), a setof laser beam folding mirrors, a light collecting/focusing mirror; and aphotodetector. The first and second laser scanning stations HST1 andHST2 are disposed opposite one another about the first rotatingpolygonal mirror PM1. Each laser scanning station generates a laserscanning beam (shown as SB1 and SB2 in FIGS. 2L and 2M) that is directedto a different point of incidence on the first rotating polygonal mirrorPM1. The incident laser beams (produced by the first and second laserscanning stations HST1 and HST2) are reflected by each facet (of thefirst polygonal mirror PM1) at varying angles as the first polygonalmirror PM1 rotates to produce two scanning beams (SB1 and SB2) whosedirection varies over the rotation cycle of the first polygonal mirrorPM1. The first and second laser scanning stations HST1 and HST2 includegroups of laser beam folder mirrors arranged about the first polygonalmirror PM1 so as to redirect the two scanning beams SB1 and SB2 tothereby generate and project different groups of laser scanning planesthrough the bottom-scanning window 16 in the top housing portion 5B′.TABLE I Mirror Positions - Horizontal Section (mm): Vertex X Y Z mh1 1115.25 18.87 3.06 2 109.09 9.19 42.85 3 99.81 69.42 40.73 4 105.97 79.100.94 5 6 7 8 mh2 1 123.91 −78.90 2.61 2 95.43 −62.89 39.73 3 95.43 3.5739.73 4 123.91 19.57 2.61 5 6 7 8 mh3 1 103.74 −140.29 25.40 2 96.02−133.84 47.43 3 99.04 −68.09 37.13 4 114.48 −80.98 −6.92 5 112.97−113.85 −1.78 6 7 8 mh4 1 62.08 −136.87 −11.25 2 66.99 −152.92 31.34 326.71 −165.23 31.34 4 21.80 −149.19 −11.25 5 6 7 8 mh5 1 −20.00 −135.31−11.19 2 −20.00 −148.24 27.91 3 −20.00 −148.24 27.91 4 −20.00 −135.31−11.19 5 6 7 8 mh6 1 −62.08 −136.87 −11.25 2 −66.99 −152.92 31.34 3−26.71 −165.23 31.34 4 −21.80 −149.19 −11.25 5 6 7 8 mh7 1 −96.02−133.84 47.43 2 −99.04 −68.09 37.13 3 −114.48 −80.98 −6.92 4 −112.97−113.85 −1.78 5 −103.74 −140.29 25.40 6 7 8 mh8 1 −123.91 −78.90 2.61 2−95.43 −62.89 39.73 3 −95.43 3.57 39.73 4 −123.91 19.57 2.61 5 6 7 8 mh91 −115.25 18.87 3.06 2 −109.09 9.19 42.85 3 −99.81 69.42 40.73 4 −105.9779.10 0.94 5 6 7 8 mh10 1 53.69 23.10 −11.94 2 14.23 28.69 8.47 3 47.5467.87 24.47 4 72.59 81.43 24.47 5 102.20 77.24 9.16 6 106.06 65.68 −1.177 83.67 39.33 −11.94 8 mh11 1 123.91 −79.28 2.61 2 75.02 −71.42 −10.49 375.02 11.97 −10.49 4 123.91 19.83 2.61 5 6 7 8 mh12 1 116.06 −105.01−10.87 2 43.62 −99.13 −10.90 3 65.09 −142.38 30.61 4 101.96 −145.3730.63 5 6 7 8 mh13 1 −101.96 −145.37 30.63 2 −65.09 −142.38 30.61 3−43.62 −99.13 −10.90 4 −116.06 −105.01 −10.87 5 6 7 8 mh14 1 −75.0211.97 −10.49 2 −75.02 −71.42 −10.49 3 −123.91 −79.28 2.61 4 −123.9119.83 2.61 5 6 7 8 mh15 1 −54.15 22.24 −10.80 2 −84.14 38.47 −10.80 3−106.53 64.81 −0.04 4 −102.66 76.38 10.30 5 −73.05 80.57 25.61 6 −48.0067.01 25.61 7 −14.70 27.83 9.60 8

TABLE II Scan Line Groups - Horizontal Section Group Mirrors ScanningStation/ Identifier in Group Scan Lines Type gh1 mh1, mh10 HST1/4vertical gh2 mh2, mh11 HST1/4 horizontal gh3 mh3, mh12 HST1/4 verticalgh4 mh4 HST1/4 horizontal mh5 HST1, HST2/8 mh6 HST2/4 gh5 mh7, mh13HST2/4 vertical gh6 mh8, HST2/4 horizontal mh14 gh7 mh9, mh15 HST2/4vertical

In addition, as shown in FIGS. 2I and 2J, the first and second laserscanning stations HST1 and HST2 each include a light collecting/focusingoptical element, e.g. parabolic light collecting mirror or parabolicsurface emulating volume reflection hologram (labeled LC_(HST1) andLC_(HST2)), that collects light from a scan region that encompasses theoutgoing scanning planes (produced by the first and second laserscanning stations HST1 and HST2) and focuses such collected light onto aphotodetector (labeled PD_(HST1) and PD_(HST2)), which produces anelectrical signal whose amplitude is proportional to the intensity oflight focused thereon. The electrical signal produced by thephotodetector is supplied to analog/digital signal processing circuitry,associated with the first and second laser scanning station HST1 andHST2, that process analog and digital scan data signals derived therefrom to perform bar code symbol reading operations as described herein.Preferably, the first and second laser scanning stations HST1 and HST2each include a laser beam production module (not shown) that generates alaser scanning beam (labeled SB1 and SB2) that is directed (preferablyby a small light directing mirror disposed in the interior of the lightcollecting/focusing element LC_(HST1) and LC_(HST2), respectively, asshown in FIGS. 2I and 2J), to a point of incidence on the first rotatingpolygonal mirror PM1.

As shown in FIGS. 2K and 2L and in tables III and IV below, the secondsection includes a second rotating polygonal mirror PM2 and a thirdscanning station (denoted VST1) that includes a laser beam productionmodule (not shown), a set of laser beam folding mirrors, a lightcollecting/focusing mirror, and a photodetector. The third laserscanning station VST1 generates a laser scanning beam (labeled as SB3 inFIG. 2M) that is directed to a point of incidence on the second rotatingpolygonal mirror PM2. The incident laser beam is reflected by each facet(of the second polygonal mirror PM2) at varying angles as the secondpolygonal mirror PM2 rotates to produce a scanning beam whose directionvaries over the rotation cycle of the second polygonal mirror PM2. Thethird laser scanning station VST1 includes a set of laser beam foldermirrors arranged about the second rotating polygonal mirror PM2 so as toredirect the scanning beam to thereby generate and project differentgroups of laser scanning planes through the side-scanning window 18.TABLE III Mirror Positions - Vertical Section (mm): Vertex X Y Z mv1 1−74.79 88.94 −10.38 2 −114.09 88.94 16.17 3 −114.09 154.82 16.17 4−74.79 154.82 −10.38 5 6 7 8 mv2 1 −61.12 131.03 −6.76 2 −77.92 146.4225.78 3 −43.75 183.72 25.78 4 −33.41 174.24 5.74 5 −31.44 163.43 −6.76 67 8 mv3 1 −29.78 160.24 −1.35 2 −34.38 185.43 27.65 3 −0.04 184.24 27.654 −0.04 159.21 −1.35 5 6 7 8 mv4 1 0.04 159.21 −1.35 2 0.04 184.24 27.653 34.38 185.43 27.65 4 29.78 160.24 −1.35 5 6 7 8 mv5 1 61.12 131.03−6.76 2 31.44 163.43 −6.76 3 33.41 174.24 5.74 4 43.75 183.72 25.78 577.92 146.42 25.78 6 7 8 mv6 1 74.79 88.94 −10.38 2 74.79 154.82 −10.383 114.09 154.82 16.17 4 114.09 88.94 16.17 5 6 7 8 mv7 1 −107.52 89.3530.99 2 −110.94 68.34 59.03 3 −136.32 120.65 95.14 4 −132.90 141.6667.10 5 6 7 8 mv8 1 −129.50 196.36 99.91 2 −139.66 144.56 68.88 3−133.18 126.69 96.58 4 −123.02 178.48 127.62 5 6 7 8 mv9 1 −42.26 185.7373.40 2 −65.99 163.92 49.03 3 −69.45 141.18 82.25 4 −45.72 162.99 106.625 6 7 8 mv10 1 0.00 190.18 78.00 2 −40.33 183.35 74.96 3 −46.98 168.27105.79 4 0.00 176.23 109.33 5 6 7 8 mv11 1 0.00 176.23 109.33 2 46.98168.27 105.79 3 40.33 183.35 74.96 4 0.00 190.18 78.00 5 6 7 8 mv12 142.26 185.73 73.40 2 45.72 162.99 106.62 3 69.45 141.18 82.25 4 65.99163.92 49.03 5 6 7 8 mv13 1 139.66 144.56 68.88 2 129.50 196.36 99.91 3123.02 178.48 127.62 4 133.18 126.69 96.58 5 6 7 8 mv14 1 132.90 141.6667.10 2 136.32 120.65 95.14 3 110.94 68.34 59.03 4 107.52 89.35 30.99 56 7 8 mv15 1 −59.72 111.27 102.01 2 −38.96 95.77 87.32 3 −42.25 116.9860.28 4 −79.46 144.76 86.61 5 −77.49 132.11 102.74 6 7 8 mv16 1 37.7388.59 93.83 2 29.22 119.90 64.12 3 −29.22 119.90 64.12 4 −37.73 88.5993.83 5 6 7 8 mv17 1 42.25 116.98 60.28 2 38.96 95.77 87.32 3 59.72111.27 102.01 4 79.46 144.76 86.61 5 42.25 116.98 60.28 6 7 8 mv18 1−63.87 149.13 93.46 2 −79.68 162.64 67.06 3 −100.06 208.14 102.55 4−84.26 194.63 128.95 5 6 7 8 mv19 1 −140.43 92.77 119.03 2 −140.43126.87 119.12 3 −136.72 174.44 128.44 4 −125.11 154.96 157.07 5 −130.4187.14 143.79 6 7 8 mv20 1 63.87 149.13 93.46 2 79.68 162.64 67.06 3100.06 208.14 102.55 4 84.26 194.63 128.95 5 6 7 8 mv21 1 130.41 87.14143.79 2 125.11 154.96 157.07 3 136.72 174.44 128.44 4 140.43 126.87119.12 5 140.43 92.77 119.03 6 7 8 mv22 1 −134.07 126.69 200.27 2−103.99 134.04 208.61 3 −94.62 209.63 108.20 4 −124.70 202.28 99.86 5 67 8 mv23 1 94.62 209.63 108.20 2 103.99 134.04 208.61 3 134.07 126.69200.27 4 124.70 202.28 99.86 5 6 7 8 mv24 1 −61.13 193.21 119.96 2−97.12 187.87 131.32 3 −97.12 169.38 170.59 4 −19.20 152.51 206.45 519.20 152.51 206.45 6 97.12 169.38 170.59 7 97.12 187.87 131.32 8 61.13193.21 119.96 mv25 1 −106.74 171.66 177.19 2 −83.23 85.77 217.46 3 0.0085.77 246.33 4 0.00 150.54 222.12 5 6 7 8 mv26 1 0.00 150.54 222.12 20.00 150.54 222.12 3 83.23 85.77 217.46 4 106.74 171.66 177.19 5 6 7 8

TABLE IV Scan Line Groups - Vertical Section Group Mirrors in ScanningStation/ Identifier Group Scan Lines Type gv1 mv1, mv22 VST1/4 verticalleft gv2 mv2, mv26 VST1/4 top-down vertical gv3 mv3, mv25 VST1/4top-down horizontal gv4 mv4, mv26 VST1/4 top-down horizontal gv5 mv5,mv25 VST1/4 top-down vertical gv6 mv6, mv23 VST1/4 vertical right gv7mv7, mv24 VST1/4 high horizontal left gv8 mv8, mv18, mv19 VST1/4 sidehorizontal left gv9 mv9, mv17, mv24 VST1/4 low horizontal left gv10mv10, mv16, mv26 VST1/4 top-down horizontal gv11 mv11, mv16, mv25 VST1/4top-down horizontal gv12 mv12, mv15, mv24 VST1/4 low horizontal rightgv13 mv13, mv20, mv21 VST1/4 side horizontal right gv14 mv14, mv24VST1/4 high horizontal right

In addition, as shown in FIG. 2M, the third laser scanning station VST1includes a light collecting/focusing optical element, e.g. paraboliclight collecting mirror or parabolic surface emulating volume reflectionhologram (labeled LC_(VST1)), that collects light from a scan regionthat encompasses the outgoing scanning planes (produced by the thirdlaser scanning station VST1) and focuses such collected light onto aphotodetector (labeled PD_(VST1)), which produces an electrical signalwhose amplitude is proportional to the intensity of light focusedthereon. The electrical signal produced by the photodetector is suppliedto analog/digital signal processing circuitry, associated with the thirdlaser scanning station VST1, that process analog and digital scan datasignals derived there from to perform bar code symbol reading operationsas described herein. Preferably, the third laser scanning station VST1includes a laser beam production module (not shown) that generates alaser scanning beam SB3 that is directed to a small light directingmirror disposed in the interior of the light collecting/focusing elementLC_(VST1), which redirects the laser scanning beam SB3 to a point ofincidence on the second rotating polygonal mirror PM2.

In the illustrative embodiment, the first polygonal mirror PM1 includes4 facets that are used in conjunction with the two independent laserbeam sources provided by the first and second laser scanning stationsHST1 and HST2 so as project from the bottom-scanning window 16 anomnidirectional laser scanning pattern consisting of 40 laser scanningplanes that are cyclically generated as the first polygonal mirror PM1rotates. Moreover, the second polygonal mirror PM2 includes 4 facetsthat are used in conjunction with the independent laser beam sourceprovided by the third laser scanning station VST1 so as to project fromthe side-scanning window an omnidirectional laser scanning patternconsisting of 28 laser scanning planes cyclically generated as thesecond polygonal mirror PM2 rotates. Thus, the bioptical laser scanningsystem of the illustrative embodiment project from the bottom andside-scanning windows 16, 18 an omnidirectional laser scanning patternconsisting of 68 laser scanning planes cyclically generated as the firstand second polygonal mirrors PM1 and PM2 rotate. It is understood,however, these number may vary from embodiment to embodiment of thepresent invention and thus shall not form a limitation thereof.

FIG. 2N 1 depicts the angle of each facet of the rotating polygonalmirrors PM1 and PM2 with respect to the rotational axis of therespective rotating polygonal mirrors in this illustrative embodiment.The scanning ray pattern produced by the four facets (as specified inFIG. 2N 1) of the first polygonal mirror PM1 in conjunction with thelaser beam source provided by the first laser scanning station HST1 isshown in FIG. 2N 2. A similar scanning ray pattern is produced by thefour facets of the first polygonal mirror PM1 in conjunction with thelaser beam source provided by the second laser scanning station HST2. Inthe illustrative embodiment of the present invention, the secondrotating polygonal mirror PM2 has two different types of facets based onbeam elevation angle characteristics of the facet. The scanning raypattern produced by the four facets of the second polygonal mirror PM2in conjunction with the laser beam source provided by the third laserscanning station VST1 is shown in FIG. 2N 3. The facets of the secondpolygonal mirror PM2 can be partitioned into two classes: a first classof facets (corresponding to angles β₁ and β₂) have High Elevation (HE)angle characteristics, and a second class of facets (corresponding toangles β₃ and β₄) have Low Elevation (LE) angle characteristics. Asshown in FIGS. 2N3, high and low elevation angle characteristics arereferenced by the plane P1 that contains the incoming laser beam and isnormal to the rotational axis of the second polygonal mirror PM2. Eachfacet in the first class of facets (having high beam elevation anglecharacteristics) produces an outgoing laser beam that is directed abovethe plane P1 as the facet sweeps across the point of incidence of thethird laser scanning station VST1. Whereas each facet in the secondclass of facets (having low beam elevation angle characteristics)produces an outgoing laser beam that is directed below the plane P1 asthe facet sweeps across the point of incidence of the third laserscanning station VST1. As will become apparent hereinafter, the use ofscanning facets having such diverse elevation angle characteristicsenables an efficient design and construction of the second section ofthe bioptical laser scanning—the plurality of beam folding mirrors usedtherein can be compactly arranged within a minimized region ofvolumetric space. Such efficient space saving designs are advantageousin space-constricted POS-type scanning applications.

In the illustrative embodiment of the present invention, the first laserscanning station (HST1) includes four groups of laser beam foldingmirrors (GH1, GH2, GH3, and GH4 as depicted in Table II above) which arearranged about the first rotating polygonal mirror PM1, and cooperatewith the four scanning facets of the first rotating polygonal mirror PM1so as to generate and project four different groups of laser scanningplanes (with 20 total scanning planes in the four groups) through thebottom-scanning window 16, as graphically illustrated in FIGS. 3A-3F2.Note that the first laser scanning station HST1 utilizes mirrors MH4 andMH5 (and not MH6) of group GH4 to produce 8 different scan planes therefrom. The second laser scanning station (HST2) includes four groups oflaser beam folding mirrors (GH4, GH5, GH6 and GH7 as depicted in TableII) which are arranged about the first rotating polygonal mirror PM1,and cooperate with the four scanning facets of the first rotatingpolygonal mirror so as to generate and project four different groups oflaser scanning planes (with 20 total scanning planes in the four groups)through the bottom-scanning window 16, as graphically illustrated inFIGS. 4A-4F. Note that the second laser scanning station HST2 utilizesmirrors MH5 and MH6 (and not MH4) of group GH4 to produce 8 differentscan planes there from. Finally, the third laser scanning station (VST1)includes fourteen groups of laser beam folding mirrors (GV1, GV2 . . .GV14 as depicted in Table IV above) which are arranged about the secondrotating polygonal mirror PM2, and cooperate with the four scanningfacets of the second rotating polygonal mirror PM2 so as to generate andproject fourteen different groups of laser scanning planes (with 28total scanning planes in the fourteen groups) through the side-scanningwindow 18, as graphically illustrated in FIGS. 5A-5P2.

For purposes of illustration and conciseness of description, each laserbeam folding mirror in each mirror group as depicted in the secondcolumn of Tables II and IV, respectively, is referred to in thesequential order that the outgoing laser beam reflects off the mirrorsduring the laser scanning plane generation process (e.g., the firstmirror in the column causes an outgoing laser beam to undergo its firstreflection after exiting a facet of the rotating polygonal mirror, thesecond mirror in the column causes the outgoing laser beam to undergoits second reflection, etc.).

First Laser Scanning Station HST1

As shown in FIGS. 2G, 2H and 3A-3F2, the first laser scanning station(HST1) includes four groups of laser beam folding mirrors (GH1, GH2, GH3and GH4) which are arranged about the first rotating polygonal mirrorPM1, and cooperate with the four scanning facets of the first rotatingpolygonal mirror PM1 so as to generate and project four different groupsof laser scanning planes (with 20 total scanning planes in the fourgroups) through the bottom-scanning window 16. The intersection of thefour groups of laser scanning planes (with 20 total scanning planes inthe four groups) on the bottom-scanning window 16 is shown in FIG. 3A.The twenty laser scanning planes (of these four groups projected throughthe bottom-scanning window 16) can be classified as either verticalscanning planes or horizontal scanning planes, which can be defined asfollows.

As shown in FIGS. 3B1 and 3B2, a scanning plane has a characteristicdirection of propagation D_(p) and a normal direction SP_(N), whichdefine a direction O that is orthogonal thereto (e.g., O=D_(p)×SP_(N)).For the sake of description, the characteristic direction of propagationD_(p) of a scanning plane can be defined as the mean propagationdirection for a plurality of rays that make up the scanning plane. Ahorizontal scanning plane is a scanning plane wherein the angle φbetween the direction O and the plane defined by the bottom-scanningwindow 16 is in the range between 0 and 45 degrees (and preferably inthe range between 0 and 20 degrees, and more preferably in the rangebetween 0 and 10 degrees). An exemplary horizontal scanning plane isshown in FIG. 3B 1. A vertical scanning plane is a scanning planewherein the angle φ between the direction O and the plane defined by thebottom-scanning window 16 is in the range between 45 and 90 degrees (andpreferably in the range between 70 and 90 degrees, and more preferablyin the range between 80 and 90 degrees). An exemplary vertical scanningplane is shown in FIG. 3B 2.

FIGS. 3C1 and 3C2 illustrate the first group GH1 of laser beam foldingmirrors of the first laser scanning station (HST1), which cooperate withthe four scanning facets of the first rotating polygonal mirror PM1 soas to generate four different vertical laser scanning planes thatproject from the right back corner of the bottom-scanning window 16diagonally outward and upward above the front left side (and front leftcorner) of the bottom-scanning window 16 as shown. These scanning planesare useful for reading ladder type bar code symbols disposed on bottom-,back-, and right-facing surfaces.

FIGS. 3D1 and 3D2 illustrate the second group GH2 of laser beam foldingmirrors of the first laser scanning station (HST1), which cooperate withthe four scanning facets of the first rotating polygonal mirror PM1 soas to generate four different horizontal laser scanning planes thatproject from the right side of the bottom-scanning window 16 diagonallyoutward and upward above the left side of the bottom-scanning window 16as shown. These scanning planes are useful for reading picket-fence typebar code symbols disposed on bottom- and right-facing surfaces.

FIGS. 3E1 and 3E2 illustrate the third group GH3 of laser beam foldingmirrors of the first laser scanning station (HST1), which cooperate withthe four scanning facets of the first rotating polygonal mirror PM1 soas to generate four different vertical laser scanning planes thatproject from the right front corner of the bottom-scanning window 16diagonally outward and upward above the back left side and back leftcorner of the bottom-scanning window 16 as shown. These scanning planesare useful for reading ladder type bar code symbols disposed on bottom-,front-, and right-facing surfaces.

FIGS. 3F1 and 3F2 illustrate the fourth group GH4 of laser beam foldingmirrors of the first laser scanning station (HST1), which cooperate withthe four scanning facets of the first rotating polygonal mirror PM1 soas to generate eight different horizontal laser scanning planes thatproject from the front side of the bottom-scanning window 16 diagonallyoutward and upward above the back side of the bottom-scanning window 16as shown. Note that the first laser scanning station HST1 utilizesmirrors MH4 and MH5 (and not MH6) of group GH4 to produce eightdifferent scan planes there from. These scanning planes are useful forreading picket-fence type bar code symbols disposed on bottom- andfront-facing surfaces.

The position and orientation of each beam folding mirror employed atscanning station HST1 relative to a global coordinate reference systemis specified by a set of position vectors pointing from the from theorigin of this global coordinate reference system to the vertices ofeach such beam folding mirror element (i.e. light reflective surfacepatch). The x,y,z coordinates of these vertex-specifying vectors as setforth above in Table I specify the perimetrical boundaries of these beamfolding mirrors employed in the scanning system of the illustrativeembodiment.

Second Laser Scanning Station HST2

As shown in FIGS. 2G, 2H and 4A-4E2, the second laser scanning station(HST2) includes four groups of laser beam folding mirrors (GH4, GH5,GH6, and GH7) which are arranged about the first rotating polygonalmirror PM1, and cooperate with the four scanning facets of the firstrotating polygonal mirror PM1 so as to generate and project fourdifferent groups of laser scanning planes (with 20 total scanning planesin the four groups) through the bottom-scanning window 16. Theintersection of the four groups of laser scanning planes (with 20 totalscanning planes in the four groups) on the bottom-scanning window 16 isshown in FIG. 4A. The twenty laser scanning planes (of these four groupsprojected through the bottom-scanning window 16) can be classified aseither vertical scanning planes or horizontal scanning planes as definedabove.

FIGS. 4B1 and 4B2 illustrate the first group (GH4) of laser beam foldingmirrors of the second laser scanning station (HST2), which cooperatewith the four scanning facets of the first rotating polygonal mirror PM1so as to generate eight different horizontal laser scanning planes thatproject from the front side of the bottom-scanning window 16 diagonallyoutward and upward above the back side of the bottom-scanning window 16as shown. Note that the second laser scanning station HST2 utilizesmirrors MH5 and MH6 (and not MH4) of group GH4 to produce eightdifferent scan planes there from. These scanning planes are useful forreading picket-fence type bar code symbols disposed on bottom- andfront-facing surfaces.

FIGS. 4C1 and 4C2 illustrate the second group (GH5) of laser beamfolding mirrors of the second laser scanning station (HST2), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate four different vertical laser scanningplanes that project from the left front corner of the bottom-scanningwindow 16 diagonally outward and upward above the back right side andback right corner of the bottom-scanning window 16 as shown. Thesescanning planes are useful for reading ladder type bar code symbolsdisposed on bottom-, front-, and left-facing surfaces.

FIGS. 4D1 and 4D2 illustrate the third group (GH6) of laser beam foldingmirrors of the second laser scanning station (HST2), which cooperatewith the four scanning facets of the first rotating polygonal mirror PM1so as to generate four different horizontal laser scanning planes thatproject from the left side of the bottom-scanning window 16 diagonallyoutward and upward above the right side of the bottom-scanning window 16as shown. These scanning planes are useful for reading picket-fence typebar code symbols disposed on bottom- and left-facing surfaces.

FIGS. 4E1 and 4E2 illustrate the fourth group (GH7) of laser beamfolding mirrors of the second laser scanning station (HST2), whichcooperate with the four scanning facets of the first rotating polygonalmirror PM1 so as to generate four different vertical laser scanningplanes that project from the left back corner of the bottom-scanningwindow 16 diagonally outward and upward above the front right side andfront right corner of the bottom-scanning window 16 as shown. Thesescanning planes are useful for reading ladder type bar code symbolsdisposed on bottom-, back-, and left-facing surfaces.

The position and orientation of each beam folding mirror employed atscanning station HST2 relative to a global coordinate reference systemis specified by a set of position vectors pointing from the from theorigin of this global coordinate reference system to the vertices ofeach such beam folding mirror element (i.e. light reflective surfacepatch). The x,y,z coordinates of these vertex-specifying vectors as setforth above in Table I specify the perimetrical boundaries of these beamfolding mirrors employed in the scanning system of the illustrativeembodiment.

As shown in FIG. 4F, the vertical scanning planes that project from thebottom-scanning window 16 include 4 groups (namely, GH1, GH3, GH5 andGH7). Groups GH1 and GH5 project from opposing portions (e.g., theback-right and front-left corners of the window 16) of thebottom-scanning window 16, and groups GH3 and GH7 project from opposingportions (e.g., front-right and back-left corners of the window 16) ofthe bottom-scanning window. Note that groups GH1 and GH5 aresubstantially co-planar (i.e., quasi co-planar) and groups GH3 and GH7are substantially co-planar (i.e., quasi co-planar), while groups GH1and GH5 are substantially orthogonal (i.e., quasi-orthogonal) to groupsGH3 and GH7, respectively, as shown.

Third Laser Scanning Station VST1

As shown in FIGS. 2K, 2L and 5A-5P2, the third laser scanning station(VST1) includes fourteen groups of laser beam folding mirrors (GV1, GV2,GV3 . . . GV14) which are arranged about the second rotating polygonalmirror PM2, and cooperate with the four scanning facets of the secondrotating polygonal mirror PM2 so as to generate and project fourteendifferent groups of laser scanning planes (with 28 total scanning planesin the fourteen groups) through the side-scanning window 18. Theintersection of the fourteen groups of laser scanning planes (with 28total scanning planes in the fourteen groups) on the side-scanningwindow 18 is shown in FIG. 5A. The twenty-eight laser scanning planes(of these fourteen groups projected through the side-scanning window 18)can be classified as either vertical scanning planes or horizontalscanning planes, which can be defined as follows.

As shown in FIG. 5B 1 and 5B2, a scanning plane has a characteristicdirection of propagation D_(p) and a normal direction SP_(N), whichdefine a direction O that is orthogonal thereto (e.g., O=D_(P)×SP_(N)).A horizontal scanning plane is a scanning plane wherein the angle φbetween the direction O and the plane defined by the bottom-scanningwindow 16 is in the range between 0 and 45 degrees (and preferably inthe range between 0 and 20 degrees, and more preferably in the rangebetween 0 and 10 degrees). An exemplary horizontal scanning planeprojected from the side-scanning window 18 is shown in FIG. 5B 1. Avertical scanning plane is a scanning plane wherein the angle φ betweenthe direction O and the plane defined by the bottom-scanning window 16is in the range between 45 and 90 degrees (and preferably in the rangebetween 70 and 90 degrees, and more preferably in the range between 80and 90 degrees). An exemplary vertical scanning plane projected from theside-scanning window 18 is shown in FIG. 5B 2.

FIGS. 5C1 and 5C2 illustrate the first group (GV1) of laser beam foldingmirrors of the third laser scanning station (VST1), which cooperate withthe two low-elevation (LE) scanning facets of the second rotatingpolygonal mirror PM2 (corresponding to angles β₃ and β₄ of the secondpolygonal mirror PM2 in FIG. 2N 1) so as to generate two differentvertical laser scanning planes that project from the left side of theside-scanning window 18 diagonally down and out across thebottom-scanning window 16 above the front right corner of thebottom-scanning window 16 as shown. These scanning planes are useful forreading ladder type bar code symbols disposed on left- and back-facingsurfaces.

FIGS. 5D1 and 5D2 illustrate the second group (GV2) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two low-elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₃ and β₄ of thesecond polygonal mirror PM2 in FIG. 2N 1) so as to generate twodifferent vertical laser scanning planes that project from the top leftcorner of the side-scanning window 18 downward toward thebottom-scanning window 16 substantially along the left side of thebottom-scanning window 16 as shown. These scanning planes are useful forreading ladder type bar code symbols disposed on top- and back-facingsurfaces.

FIGS. 5E1 and 5E2 illustrate the third group (GV3) of laser beam foldingmirrors of the third laser scanning station (VST1), which cooperate withthe two low-elevation scanning facets of the second rotating polygonalmirror PM2 (corresponding to angles β₃ and β₄ of the second polygonalmirror PM2 in FIG. 2N 1) so as to generate two different horizontallaser scanning planes that project from the top left quadrant of theside-scanning window 18 diagonally down across the bottom-scanningwindow 16 as shown. These scanning planes are useful for readingpicket-fence type bar code symbols disposed on back- and top-facingsurfaces.

FIGS. 5F1 and 5F2 illustrate the fourth group (GV4) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two low elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₃ and β₄ of thesecond polygonal mirror PM2 in FIG. 2N 1) so as to generate twodifferent horizontal laser scanning planes that project from the topright quadrant of the side-scanning window 18 diagonally down across thebottom-scanning window 16 as shown. These scanning planes are useful forreading picket-fence type bar code symbols disposed on back- andtop-facing surfaces.

FIGS. 5G1 and 5G2 illustrate the fifth group (GV5) of laser beam foldingmirrors of the third laser scanning station (VST1), which cooperate withthe two low-elevation scanning facets of the second rotating polygonalmirror PM2 (corresponding to angles β₃ and β₄ of the second polygonalmirror PM2 in FIG. 2N 1) so as to generate two different vertical laserscanning planes that project from the top right corner of theside-scanning window 18 downward toward the bottom-scanning window 16substantially along the right side of the bottom-scanning window 16 asshown. These scanning planes are useful for reading ladder type bar codesymbols disposed on top- and back-facing surfaces.

FIGS. 5H1 and 5H2 illustrate the sixth group (GV6) of laser beam foldingmirrors of the third laser scanning station (VST1), which cooperate withthe two low elevation scanning facets of the second rotating polygonalmirror PM2 (corresponding to angles β₃ and β₄ of the second polygonalmirror PM2 in FIG. 2N 1) so as to generate two different vertical laserscanning planes that project from the right side of the side-scanningwindow 18 diagonally out across the bottom-scanning window 16 above thefront left corner of the bottom-scanning window 16 as shown. Thesescanning planes are useful for reading ladder type bar code symbolsdisposed on right- and back-facing surfaces.

FIGS. 5I1 and 5I2 illustrate the seventh group (GV7) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2N 1) so as to generate twodifferent horizontal laser scanning planes that project from the topleft quadrant of the side-scanning window 18 outwardly across thebottom-scanning window 16 (substantially parallel to the bottom-scanningwindow 16) as shown. These scanning planes are useful for readingpicket-fence type bar code symbols disposed on back- and left-facingsurfaces.

FIGS. 5J1 and 5J2 illustrate the eight group (GV8) of laser beam foldingmirrors of the third laser scanning station (VST1), which cooperate withthe two high elevation scanning facets of the second rotating polygonalmirror PM2 (corresponding to angles β₁ and β₂ of the second polygonalmirror PM2 in FIG. 2N 1) so as to generate two different horizontallaser scanning planes that project from the left side of theside-scanning window 18 outwardly across the bottom-scanning window 16(substantially parallel to the bottom-scanning window 16) as shown. Inthe illustrative embodiment, the characteristic direction of propagationof such scanning planes has a non-vertical component (i.e., componentsin the plane parallel to the bottom-scanning window 16) whoseorientation relative to the normal of the side-scanning window 18 isgreater than 35 degrees. These scanning planes are useful for readingpicket-fence type bar code symbols disposed on back- and left-facingsurfaces (including those surfaces whose normals are substantiallyoffset from the normal of the side-scanning window).

FIGS. 5K1 and 5K2 illustrate the ninth group (GV9) of laser beam foldingmirrors of the third laser scanning station (VST1), which cooperate withthe two high elevation scanning facets of the second rotating polygonalmirror PM2 (corresponding to angles 131 and 132 of the second polygonalmirror PM2 in FIG. 2N 1) so as to generate two different horizontallaser scanning planes that project from the central portion of theside-scanning window 18 outwardly and downward across thebottom-scanning window 16 as shown. These scanning planes are useful forreading picket-fence type bar code symbols disposed on back-facingsurfaces.

FIGS. 5L1 and 5L2 illustrate the tenth group (GV10) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2N 1) so as to generate twodifferent horizontal laser scanning planes that project from the centralportion of the side-scanning window 18 outwardly and sharply downwardacross the bottom-scanning window 16 as shown. These scanning planes areuseful for reading picket-fence type bar code symbols disposed on top-and back-facing surfaces.

FIGS. 5MI and 5M2 illustrate the eleventh group (GV11) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2N 1) so as to generate twodifferent horizontal laser scanning planes that project from the centralportion of the side-scanning window 18 outwardly and sharply downwardacross the bottom-scanning window 16 as shown. These scanning planes areuseful for reading picket-fence type bar code symbols disposed on top-and back-facing surfaces.

FIGS. 5N1 and 5N2 illustrate the twelfth group (GV12) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2N 1) so as to generate twodifferent horizontal laser scanning planes that project from the centralportion of the side-scanning window 18 outwardly across thebottom-scanning window 16 (substantially parallel to the bottom-scanningwindow 16) as shown. These scanning planes are useful for readingpicket-fence type bar code symbols disposed on back-facing surfaces.

FIGS. 5O1 and 5O2 illustrate the thirteenth group (GV13) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2N 1) so as to generate twodifferent horizontal laser scanning planes that project from the rightside of the side-scanning window 18 outwardly across the bottom-scanningwindow 16 (substantially parallel to the bottom-scanning window 16) asshown. In the illustrative embodiment, the characteristic direction ofpropagation of such scanning planes has a non-vertical component (i.e.,components in the plane parallel to the bottom-scanning window 16) whoseorientation relative to the normal of the side-scanning window 18 isgreater than 35 degrees. These scanning planes are useful for readingpicket-fence type bar code symbols disposed on back- and right-facingsurfaces (including those surfaces whose normals are substantiallyoffset from the normal of the side-scanning window).

FIGS. 5P1 and 5P2 illustrate the fourteenth group (GV14) of laser beamfolding mirrors of the third laser scanning station (VST1), whichcooperate with the two high elevation scanning facets of the secondrotating polygonal mirror PM2 (corresponding to angles β₁ and β₂ of thesecond polygonal mirror PM2 in FIG. 2N 1) so as to generate twodifferent horizontal laser scanning planes that project from the topright quadrant of the side-scanning window 18 outwardly across thebottom-scanning window 16 (substantially parallel to the bottom-scanningwindow 16) as shown. These scanning planes are useful for readingpicket-fence type bar code symbols disposed on back- and right-facingsurfaces.

The position and orientation of each beam folding mirror employed atscanning station VST1 relative to a global coordinate reference systemis specified by a set of position vectors pointing from the from theorigin of this global coordinate reference system to the vertices ofeach such beam folding mirror element (i.e. light reflective surfacepatch). The x,y,z coordinates of these vertex-specifying vectors as setforth above in Table III specifies the perimetrical boundaries of thesebeam folding mirrors employed in the scanning system of the illustrativeembodiment.

In the illustrative bioptical laser scanning system, the laser beamfolding mirrors associated with scanning stations HST1, HST2 and VST1are physically supported utilizing one or more mirror support platforms,formed with the scanner housing. Preferably, these mirror mountingsupport structures, as well as the components of the scanning housingare made from a high-impact plastic using injection molding techniqueswell known in the art.

TDM Laser Scanning

In the illustrative bioptical laser scanning system, the principalfunction of each facet on the first and second rotating polygonalmirrors PM1 and PM2 is to deflect an incident laser beam along aparticular path in 3-D space in order to generate a correspondingscanning plane within the 3-D laser scanning volume produced by thelaser scanning system hereof. Collectively, the complex of laserscanning planes produced by the plurality of facets in cooperation withthe three laser beam production modules of HST1, HST2 and VST1 createsan omnidirectional scanning pattern within the highly-defined 3-Dscanning volume of the scanning system between the space occupied by thebottom and side-scanning windows of the system. As shown in theexemplary timing scheme of FIG. 6A, the illustrative bioptical laserscanner cyclically generates a complex omnidirectional laser scanningpattern from both the bottom and side-scanning windows 16 and 18 thereofin a time-division-multiplexed manner during the revolutions of thescanning polygonal mirrors PM1 and PM2. In this exemplary timing scheme,four sets of scan plane groups (4*[GH1 . . . GH7]) are produced bystations HST1 and HST2 during each revolution of the polygonal mirrorPM1. Moreover, two sets of scan plane groups (2*[GV1 . . . GV14]) areproduced by station VST1 during a single revolution of the polygonalmirror PM2. The complex omnidirectional TDM scanning pattern isgraphically illustrated in FIGS. 3A through 5P2, which consists of 68different TDM laser scanning planes which cooperate in order to generatea plurality of quasi-orthogonal TDM laser scanning patterns within the3-D scanning volume of the system, thereby enabling true omnidirectionalscanning of bar code symbols. In the TDM timing scheme of FIG. 6A, theTDM laser scanning operations of the scanning planes produced by HST1,HST2 and VST1 are defined by a plurality of time slots uniquely assignedto HST1, HST2, VST1, respectively, over each scanning plane producedtherefrom. In other words, the scan period of a given scanning planeincludes a plurality of time slots logically assigned to the laserscanning station (HST1 or HST2 or VST1) that produces the given scanningplane. In this manner, the TDM laser scanning operations (andcorresponding signal processing operations) over given scanning planeare performed at high frequencies (for example, frequencies greater thantwo times the Nyquist frequency, and possibly at frequencies greaterthan four times the Nyquist frequency).

FIG. 6B illustrates an alternate timing scheme for controlling thebioptical laser scanner of the illustrative embodiment to cyclicallygenerate a complex omni-directional TDM laser scanning pattern from boththe bottom and side-scanning windows 16 and 18 thereof during therevolutions of the scanning polygonal mirrors PM1 and PM2. Thisalternative timing scheme is similar to the timing scheme of FIG. 6A inthat four sets of scan plane groups (4*[GH1 . . . GH7]) are produced bystations HST1 and HST2 during each revolution of the polygonal mirrorPM1, and two sets of TDM scan plane groups (2*[GV1 . . . GV14]) areproduced by station VST1 during a single revolution of the polygonalmirror PM2. However, this complex omni-directional scanning pattern isproduced in a time-division-multiplexed manner different from FIG. 6A.More specifically, in FIG. 6B, the TDM laser scanning operations of thescanning planes produced by HST1 and HST2 are defined by time slotsuniquely assigned to HST1 and HST2, respectively, which correspond toscanning planes produced by HST1 and HST2. In other words, the scanperiod of a given scanning plane corresponds to a time slot logicallyassigned to the laser scanning station (HST1 or HST2) that produces thegiven scanning plane.

The TDM laser scanning operations of FIG. 6B include the followingoperations. During the time slots (e.g., scanning plane periods)logically assigned to station HST1, the generation and/or projection ofthe laser scanning planes from station HST1 and the signal processingoperations (for bar code detection) on the output of the photosensorPD_(HST1) are synchronously enabled, and the generation and/orprojection of the laser scanning planes from station HST2 and the signalprocessing operations on the output of the photosensor PD_(HST2) aredisabled (e.g., turned OFF). Similarly, during the time slots (e.g.,scanning plane periods) logically assigned to station HST2, thegeneration and/or projection of the laser scanning planes from stationHST2 and the signal processing operations (for bar code detection) onthe output of the photosensor PD_(HST2) are synchronously enabled, andthe generation and/or projection of the laser scanning planes fromstation HST1 and the signal processing operations (for bar codedetection) on the output of the photosensor PD_(HST1) are disabled(e.g., turned OFF). In this manner, the TDM laser scanning operations(and corresponding signal processing operation) are performed at lowerfrequencies (e.g., much less than the Nyquist frequency).

FIGS. 6C1 through 6C3 illustrates alternate timing schemes forcontrolling the illustrative bioptical laser scanner to cyclicallygenerate a complex omni-directional laser scanning pattern from both thebottom and side-scanning windows 16 and 18 thereof during therevolutions of the scanning polygonal mirrors PM1 and PM2. Thesealternative timing schemes are similar to the timing scheme of FIGS. 6Aand 6B in that four sets of scan plane groups (4*[GH1 . . . GH7]) areproduced by stations HST1 and HST2 during each revolution of thepolygonal mirror PM1, and two sets of TDM scan plane groups (2*[GV1 . .. GV14]) are produced by station VST1 during a single revolution of thepolygonal mirror PM2. However, this complex omni-directional scanningpattern is produced in a time-division-multiplexed manner different fromFIGS. 6A and 6B. More specifically, in FIGS. 6C1-6C3, the TDM laserscanning operations of the scanning planes produced by HST1 and HST2 aredefined by time slots uniquely assigned to HST1 and HST2, respectively,which correspond to scanning plane groups produced by HST1 and HST2. Inother words, the scan period of a given scanning plane group correspondsto a time slot logically assigned to the laser scanning station (HST1 orHST2) that produces the given scanning plane group.

The TDM laser scanning operations of FIGS. 6C1-6C3 include the followingoperations During the time slots (e.g., scanning plane group periods)logically assigned to station HST1, the generation and/or projection ofthe laser scanning planes from station HST1 and the signal processingoperations (for bar code detection) on the output of the photosensorP_(DHST1) are synchronously enabled, and the generation and/orprojection of the laser scanning planes from station HST2 and the signalprocessing operations (for bar code detection) on the output of thephotosensor PD_(HST2) are disabled (e.g., turned OFF). Similarly, duringthe time slots (e.g., scanning plane group periods) logically assignedto station HST2, the generation and/or projection of the laser scanningplanes from station HST2 and the signal processing operations (for barcode detection) on the output of the photosensor PD_(HST2) aresynchronously enabled, and the generation and/or projection of the laserscanning planes from station HST1 and the signal processing operations(for bar code detection) on the output of the photosensor PD_(HST2) aredisabled (e.g., turned OFF. In this manner, the TDM laser scanningoperations (and corresponding signal processing operation) are performedat lower frequencies (e.g., much less than the Nyquist frequency).

In the TDM laser scanning scheme of FIG. 6C 1, the TDM time slots arelogically assigned to scanning plane groups produced by stations HST1and HST2 during a_revolution of the polygonal mirror PM1.

In the TDM laser scanning scheme of FIG. 6C 2, the TDM time slots arelogically assigned to scanning plane groups produced by stations HST1and HST2 during a_revolution of the polygonal mirror PM1.

And in TDM laser scanning scheme of FIG. 6C 3, the TDM time slots arelogically assigned to scanning plane groups produced by stations HST1and HST2 during a full revolution of the polygonal mirror PM1.

Note that in the TDM laser scanning schemes described above with respectto FIGS. 6B, 6C1, 6C2 and 6C3, the third laser scanning station VST1 isnot part of the TDM laser scanning and signal processing operationsperformed by the first and second laser scanning stations HST1 and HST2.In other alternate timing schemes, the third laser scanning station VST1may perform TDM laser scanning and signal processing operations withrespect to the laser scanning and signal processing operations of thefirst and second laser scanning stations HST1 and HST2. For example, thetiming schemes of FIGS. 6B, 6C1, 6C2 and 6C3 may be modified such thattime-division-multiplexed laser scanning and signal processingoperations (bar code detection) are performed between the third laserscanning station VST1 and the active HST laser scanning station (i.e.,station HTS1 or HST2).

In order to allow the illustrative laser scanning system to selectivelyactivate/deactivate the laser scanning operations and signal processingoperations between scanning planes (and scanning plane groups), asynchronizing signal can be used to identify the period of time that aparticular laser scanning plane/laser scanning plane group is produced.This synchronizing signal can be derived from a position sensor (such asa hall sensor), integrated into the rotating shaft (or other portion) ofthe rotating polygonal mirror, that generates an electrical signal whenthe rotating polygonal mirror reaches a predetermined point (such as astart-of-scan position) in its rotation. Alternatively, suchsynchronization may be derived from a position indicating opticalelement (e.g., mirror or lens), which is preferably mounted adjacent (ornear) the perimeter of one of the light folding mirrors, such that theposition indicating optical element is illuminated by the scanning beamwhen the rotating polygonal mirror reaches a predetermined point (suchas a start-of-scan position) in its rotation. The position indicatingoptical element may be a mirror that directs the illumination of thescanning beam incident thereon to a position indicating optical detector(which generates an electrical signal whose amplitude corresponds to theintensity of light incident thereon). Alternatively, the positionindicating optical element may be a light collecting lens that isoperably coupled to a light guide (such as a fiber optic bundle) thatdirects the illumination of the scanning beam incident thereon to aposition indicating optical detector (which generates an electricalsignal whose amplitude corresponds to the intensity of light incidentthereon).

Moreover, in some scanning applications, where omni-directional scanningcannot be ensured at all regions within a pre-specified scanning volume,it may be useful to use scan data produced either (i) from the samelaser scanning plane reproduced many times over a very short timeduration while the code symbol is being scanned therethrough, or (ii)from several different scanning planes spatially contiguous within apre-specified portion of the scanning volume. In the first instance, ifthe bar code symbol is moved through a partial region of the scanningvolume, a number of partial scan data signal fragments associated withthe moved bar code symbol can be acquired by a particular scanning planebeing cyclically generated over an ultra-short period of time (e.g. 1-3milliseconds), thereby providing sufficient scan data to read the barcode symbol. In the second instance, if the bar code symbol is withinthe scanning volume, a number of partial scan data signal fragmentsassociated with the bar code symbol can be acquired by several differentscanning planes being simultaneously generated by the three laserscanning stations of the system hereof, thereby providing sufficientscan data to read the bar code symbol, that is, provided such scan datacan be identified and collectively gathered at a particular decodeprocessor for symbol decoding operations.

In order to allow the illustrative bioptical scanning system to usesymbol decoding algorithms that operate upon partial scan data signalfragments, as described above, the synchronization signal (as describedabove) can be used to identify a set of digital word sequences D₃, (i.e.{D_(s)}), associated with a set of time-sequentially generated laserscanning beams produced by a particular facet on the first and secondrotating polygonal mirrors. In such applications, each set of digitalword sequences can be used to decode a partially scanned code symbol andproduce symbol character data representative of the scanned code symbol.In code symbol reading applications where complete scan data signals areused to decode scanned code symbols, the synchronizing signal describedabove need not be used, as the digital word sequence D₃ corresponding tothe completely scanned bar code symbol is sufficient to carry out symboldecoding operations using conventional symbol decoding algorithms knownin the art.

As each synchronizing pulse in the synchronizing signal is synchronouswith a “reference” point on the respective rotating mirror, the symboldecoding circuitry provided with this periodic signal can readily “linkup” or relate, on a real-time basis, such partial scan data signalfragments with the particular facet on the respective rotating polygonalmirror that generated the partial scan data fragment. By producing botha scan data signal and a synchronizing signal as described above, thelaser scanning system of the present invention can readily carry out adiverse repertoire of symbol decoding processes which use partial scandata signal fragments during the symbol reading process.

In each laser scanning station (HST1, HST2, and VST1) of theillustrative embodiment, a laser beam production module produces a laserbeam that is directed at the point of incidence on the facets of thefirst or second rotating polygonal mirrors at the pre-specified angle ofincidence. Preferably, such laser beam production modules comprise avisible laser diode (VLD) and possibly an aspheric collimating lenssupported within the bore of a housing mounted upon the optical bench ofthe module housing.

In the illustrative embodiment described above, the pre-specified angleof incidence of the laser beams produced by the laser beam productionmodules of the laser scanning stations HST1 and HST2 are offset from therotational axis of the polygonal mirror PM1 along a directionperpendicular to the rotational axis as shown in FIG. 20. Such offsetprovides for spatial overlap in the scanning pattern of light beamsproduced from the polygonal mirror PM1 by these laser beam productionmodules. In the illustrative embodiment, the offset between therotational axis of the rotating polygonal mirror PM1 and the incidentdirections of the scanning beams SB1 and SB2, respectively, isapproximately 5 mm. Such spatial overlap can be exploited such that theoverlapping rays are incident on at least one common mirror (mh5 in theillustrative embodiment) to provide a dense scanning pattern projectingthere from. In the illustrative embodiment, a dense pattern ofhorizontal planes (groups GH4) is projected from the front side of thebottom window as is graphically depicted in FIGS. 3F1, 3F2 and 4B1 and4B2.

Light Collection for the 3 Scanning Stations

When a bar code symbol is scanned by any one of the laser scanningplanes projected from the bottom-scanning window 16 (by either the firstor second laser scanning stations HST1, HST2), or by any one of thelaser scanning planes projected from the side-scanning window 18 by thethird laser scanning station VST1, the incident laser light scannedacross the object is intensity modulated by the absorptive properties ofthe scanned object and scattered according to Lambert's Law (for diffusereflective surfaces). A portion of this laser light is reflected backalong the outgoing ray (optical) path, off the same group of beamfolding mirrors employed during the corresponding laser beam generationprocess, and thereafter is incident on the same scanning facet (of thefirst or second rotating polygonal mirror) that generated thecorresponding scanning plane only a short time before. The scanningfacet directs the returning reflected laser light towards a lightcollecting optical element (e.g., parabolic mirror structure) of therespective laser scanning station, which collects the returning lightand focuses these collected light rays onto a photodetector, which maybe disposed on a planar surface beneath the respective scanning polygon(as shown in FIGS. 2I and 2J), or which may be disposed on a planarsurface above the respective scanning polygon (as shown in FIG. 2M).FIGS. 2I and 2J depict the light collection optical elements LC_(HST1)and LC_(HST2), e.g., parabolic mirrors, and photodetectors PD_(HST1) andPD_(HST2) for the two laser scanning stations HST1 and HST2,respectively. FIG. 2M depicts the light collection optical elementsLC_(VST1), e.g., parabolic mirror, and photodetector PD_(VST1) for thethird laser scanning station VST1. The electrical signal produced by thephotodetector for the respective laser scanning stations is supplied toanalog/digital signal processing circuitry, associated with therespective laser scanning stations, that process analog and digital scandata signals derived there from to perform bar code symbol readingoperations as described herein.

The bottom and side-scanning windows 16 and 18 have light transmissionapertures of substantially planar extent. In order to seal off theoptical components of the scanning system from dust, moisture and thelike, the scanning windows 16 and 18, are preferably fabricated from ahigh impact plastic material and installed over their correspondinglight transmission apertures using a rubber gasket and conventionalmounting techniques. In the illustrative embodiment, each scanningwindow 16 and 18 preferably has spectrally-selective light transmissioncharacteristics which, in conjunction with a spectrally-selectivefilters installed before each photodetector within the housing, forms anarrow-band spectral filtering subsystem that performs two differentfunctions. The first function of the narrow-band spectral filteringsubsystem is to transmit only the optical wavelengths in the red regionof the visible spectrum in order to impart a reddish color orsemi-transparent character to the scanning window. This makes theinternal optical components less visible and thus remarkably improvesthe external appearance of the bioptical laser scanning system. Thisfeature also makes the bioptical laser scanner less intimidating tocustomers at point-of-sale (POS) stations where it may be used. Thesecond function of the narrow-band spectral filtering subsystem is totransmit to the photodetector for detection, only the narrow band ofspectral components comprising the outgoing laser beam produced by theassociated laser beam production module. Details regarding this opticalfiltering subsystem are disclosed in copending application Ser. No.08/439,224, entitled “Laser Bar Code Symbol Scanner Employing OpticalFiltering With Narrow Band-Pass Characteristics and Spatially SeparatedOptical Filter Elements” filed on May 11, 1995, which is incorporatedherein by reference in its entirety.

Electrical Subsystem

The illustrative bioptical laser scanning system 1′ comprises a numberof system components as shown in the system diagram of FIG. 7,including: photodetectors (e.g. a silicon photocells) for detection ofoptical scan data signals generated by the respective laser scanningstations (e.g., HST1, HST2, VST1); signal conditioning circuitry forconditioning (e.g., preamplification and/or filtering out unwanted noisein) the electrical signals out by the photodetectors; bar code symboldetection circuitry (e.g., analog and/or digital circuitry) that forms adigitized representation (e.g., a sequence of binary bit values) of abar code label being read from signals derived from the output of thesignal conditioning circuitry; bar code digitization circuitry thatconverts the digitized representation of the bar code symbol being readinto a corresponding digital word value, and bar code symbol decodecircuitry that decodes the digital word value of the bar code symbolbeing read to generate character data string values associatedtherewith.

As described above, during laser scanning operations, the optical scandata signal D₀ focused and incident on the photodetectors is produced bylight rays associated with a diffracted laser beam being scanned acrossa light reflective surface (e.g. the bars and spaces of a bar codesymbol) and scattering thereof, whereupon the polarization statedistribution of the scattered light rays is typically altered when thescanned surface exhibits diffuse reflective characteristics. Thereafter,a portion of the scattered light rays are reflected along the sameoutgoing light ray paths toward the facet which produced the scannedlaser beam. These reflected light rays are collected by the scanningfacet and ultimately focused onto the photodetector by its paraboliclight reflecting mirror. The function of each photodetector is to detectvariations in the amplitude (i.e. intensity) of optical scan data signalD₀, and produce in response thereto an electrical analog scan datasignal D₁ which corresponds to such intensity variations. When aphotodetector with suitable light sensitivity characteristics is used,the amplitude variations of electrical analog scan data signal D₁ willlinearly correspond to light reflection characteristics of the scannedsurface (e.g. the scanned bar code symbol). The function of the signalconditioning circuitry is to amplify and/or filter the electrical analogscan data signal D₁, in order to improve the SNR of the analog signal.

The bar code symbol detection circuitry processes the conditioned D₁signals produced by the signal conditioning circuitry to form adigitized representation (e.g., a sequence of binary bit values) of abar code label being read from the information encoded in theconditioned D₁ signals. In practice, this processing (which may beperformed in the analog domain or digital domain) is a thresholdingfunction which converts the conditioned analog scan data signal D₁ intoa corresponding digital scan data signal D₂ having first and second(i.e. binary) signal levels which correspond to the bars and spaces ofthe bar code symbol being scanned. Thus, the digital scan data signal D₂appears as a pulse-width modulated type signal as the first and secondsignal levels vary in proportion to the width of bars and spaces in thescanned bar code symbol.

The bar code digitization circuitry processes the digital scan datasignal D₂, associated with each scanned bar code symbol, to form acorresponding sequence of digital words D₃ (i.e., a sequence of digitalcount values). Notably, in the digital word sequence D₃, each digitalword represents the time length associated with each first or secondsignal level in the corresponding digital scan data signal D₂.Preferably, these digital count values are in a suitable digital formatfor use in carrying out various symbol decoding operations which, likethe scanning pattern and volume of the present invention, will bedetermined primarily by the particular scanning application at hand.Reference is made to U.S. Pat. No. 5,343,027 to Knowles, incorporatedherein by reference, as it provides technical details regarding thedesign and construction of microelectronic bar code digitizationcircuits suitable for use in the illustrative bioptical laser scanningsystem.

The bar code symbol decoding circuitry receive the digital wordsequences D₃ produced from the bar code digitization circuits, andsubject such words to one or more bar code symbol decoding algorithms inorder to determine which bar code symbol is indicated (i.e. represented)by the given digital word sequence D₃, originally derived fromcorresponding scan data signal D₁ detected by the photodetectorassociated therewith. In more general scanning applications, thefunction of the bar code symbol decoding circuitry is to receive eachdigital word sequence D₃ produced from the digitizing circuit, andsubject it to one or more pattern recognition algorithms (e.g. characterrecognition algorithms) in order to determine which pattern is indicatedby the digital word sequence D₃. In bar code symbol readingapplications, in which scanned code symbols can be any one of a numberof symbologies, a bar code symbol decoding algorithm withauto-discrimination capabilities can be used in a manner known in theart. In the preferred embodiment, the bar code symbol decoding functionis carried out in software as part of a programmed routine that executeson the programmed microprocessor.

Details of exemplary signal processing circuitry for signal conditioningand bar code detection and decoding is described below.

As shown in FIG. 7, the system also includes a programmed microprocessor(e.g., system controller) with a system bus and associated program anddata storage memory, for controlling the system operation of thebioptical laser scanner and performing other auxiliary functions and forreceiving bar code symbol character data (provided by the bar codesymbol decoding circuitry); a data transmission subsystem forinterfacing with and transmitting symbol character data and otherinformation to host computer system (e.g. central computer, cashregister, etc.) over a communication link therebetween; and aninput/output interface for providing drive signals to anaudio-transducer and/or LED-based visual indicators used to signalsuccessful symbol reading operations to users and the like, forproviding user input via interaction with a keypad, and for interfacingwith a plurality of accessory devices (such as an external handheldscanner that transmits bar code symbol character data to the biopticallaser scanning system, a display device, a weight scale, a magnetic cardreader and/or a coupon printer as shown). In addition, the input-outputinterface may provide a port that enables an external handheld scannerto transmit sequences of digital words D₃ (i.e. a sequence of digitalcount values) generated therein to the bioptical laser scanning systemfor bar code symbol decoding operations. Details of such an interfaceport are described in U.S. Pat. No. 5,686,717 to Knowles et al.,commonly assigned to the assignee of the present invention, hereinincorporated by reference in its entirety.

The microprocessor also produces motor control signals, and lasercontrol signals during system operation. Motor control circuitryoperates in response to such motor control signals to drive the twomotors (motor 1 and motor 2) that cause rotation of the first and secondrotating polygonal mirrors PM1 and PM2, respectively. A power regulationcircuit receives 120 Volt, 60 Hz line voltage signal from an externalpower source (such as a standard power distribution circuit) andprovides a regulated supply of electrical power to electrical componentsof the scanning system.

The communication link between the data transmission subsystem and thehost system may be a wireless data link (such as an infra-red link,Bluetooth RF link or IEEE 802.11a or 802.11b RF link) or wired serialdata link (such as keyboard wedge link—for example supporting XT-, AT-and PS/2-style keyboard protocols, an RS-232 link, USB link, a Firewire(or IEEE 1394) link, an RS-422 link, and RS-485 link), a wired paralleldata bus, or other common wired interface links (such as an OCIA link,IBM 46XX link, Light Pen Emulation link, LTPN link). Similarly, theinput/output interface between the external handheld scanner and thebioptical laser scanning system may support a wireless data link (suchas an infra-red link, Bluetooth RF link or IEEE 802.11a or 802.11b RFlink) or wired serial data link (such as keyboard wedge link—for examplesupporting XT-, AT- and PS/2-style keyboard protocols, an RS-232 link,USB link, a Firewire (or IEEE 1394) link, an RS-422 link, and RS-485link), a wired parallel data bus, or other common wired interface links(such as an OCIA link, IBM 46XX link, Light Pen Emulation link, LTPNlink).

The VLD control circuitry operates in response to the laser controlsignals supplied by the microprocessor to generate timing signalsHST1_ENABLE, HST2_ENABLE and VST1_ENABLE (as described above) thatdefine time slots that are logically assigned to unique VLD modules(HST1 VLD, HST2 VLD or VST1 VLD) and corresponding photosensors(PD_(HST1), PD_(HST2), or PD_(VST1)). One skilled in the electronic artswill understand that there are many different ways to realize such VLDcontrol circuitry. An exemplary implementation of such VLD controlcircuitry is described below with respect to FIGS. 8A, 8B1 through 8B9,and 8C.

The modulation enabled VLD drive circuitry operates in response to thetiming signals generated by the VLD control circuitry to modulate theoutput optical power of the three VLD devices (HST1 VLD, HST2 VLD andVST1 VLD) such that, during each given time slot, the VLD logicallyassigned thereto is turned ON and projected into the scanning volumewhile the other VLDs (those not logically assigned to the given timeslot) are turned substantially OFF. One skilled in the art willunderstand that there are many different ways to realize such modulationenable VLD drive circuitry. Exemplary implementations of such modulationenable VLD drive circuitry is described below with respect to FIGS. 8Dthrough 8G2.

In addition, the signal processing circuitry that performs signalconditioning and/or bar code detection operations on the data signalsderived from the three photosensors (PD_(HST1), PD_(HST2), andPD_(VST1)) operates in response to the timing signals generated by theVLD control circuitry (in a synchronous manner with the modulationenabled VLD drive circuitry) such that, during each given time slot (ora portion of the given time slot), the photosensor logically assignedthereto is operably coupled to the signal processing circuitry thatperforms bar code detection operations on the data signals derivedtherefrom. Furthermore, during the given time slot, the otherphotosensor is operably decoupled (e.g., electrically isolated) fromsuch bar code detection signal processing circuitry.

The time-division-multiplexed laser scanning operations andcorresponding signal processing operations carried out by the biopticalscanner of FIG. 7 may be performed at high frequencies (for example,frequencies greater than two times the Nyquist frequency, and possiblyat frequencies greater than four times the Nyquist frequency asdescribed herein). In such embodiments, when a given photosensor isoperably decoupled (e.g., electrically isolated) from bar code detectionsignal processing circuitry, such bar code detection signal processingcircuitry operates on a sample of the data signals derived from thegiven photosensor during the previous time slot logically assigned tothe given photosensor. Such operations are preferably provided by trackand hold circuitry (or sample and hold circuitry which is well known inthe electronic arts) which operably couples/decouples (e.g.,electrically isolates) the output of a given photosensor from bar codedetection signal processing circuitry, and stores a sample of the datasignals derived from the given photosensor during the previous time slotlogically assigned to the given photosensor on a hold capacitor that isoperably coupled to bar code detection signal processing circuitry.

Alternatively, the time-division-multiplexed laser scanning operationsand corresponding signal processing operations carried out by thebioptical scanner of FIG. 7 may be performed at lower frequencies (e.g.,much less than the Nyquist frequency as described herein). In suchembodiments, a multiplexing switch may be used to operablycouple/decouple (e.g., electrically isolate) the output of a givenphotosensor from bar code detection signal processing circuitry, andthere is no need to store (and process) a sample of the data signalsderived from the given photosensor during the previous time slotlogically assigned to the given photosensor.

One skilled in the art will understand that there are many differentways to realize signal processing circuitry that performs thetime-division multiplexed signal processing operations on the datasignals derived from the three photosensors. An exemplary implementationof such signal processing circuitry is described below with respect toFIG. 9A, wherein bar code detection is carried out in the analog domainby analog signal processing circuitry. Exemplary implementations of suchanalog signal processing circuitry is described below with respect toFIGS. 11A through 18.

Alternatively, such bar code detection operations can be carried out inthe digital domain by digital signal processing circuitry as illustratedin FIGS. 9B and 9C. In such embodiments, analog-to-digital conversioncircuitry samples/converts analog scan data signals derived from theoutput of three photosensors PD_(HST1), PD_(HST2), and PD_(VST1) intodigital scan data signals in response to the timing signals generated bythe VLD control circuitry. Such operations are performed in asynchronous manner with the modulation enabled VLD drive circuitry suchthat, during a portion of a given time slot, the photosensor logicallyassigned thereto is operably coupled to analog-to-digital conversioncircuitry that samples/converts the analog scan data signal from thephotosensor to digital scan data signals. Such digital scan data signalsare provided to digital signal processing circuitry that performs barcode detection operations on the digital scan data signals. Furthermore,during the given time slot, the other photosensors are operablydecoupled (e.g., electrically isolated) from the analog-to-digitalconversion circuitry, and the bar code detection digital signalprocessing circuitry operates on digital data signals derived from theoutput of the other photosensors during the previous time slotslogically assigned thereto. One skilled in the art will understand thatthere are many different ways to realize such digital signal processingcircuitry. Exemplary implementations of such digital signal processingcircuitry is described below with respect to FIGS. 19A-19D.

Advantageously, such synchronous time-division multiplexed laserscanning and signal processing operations enable the illustratebioptical laser scanning system to scan the scanning volume with aplurality of multi-dimensional laser scanning beams to detect and decodebar code symbols on surfaces disposed therein while avoiding opticalcrosstalk in addition to light interference from ambient light andunwanted reflections.

Modifications

The illustrative laser scanning systems described herein can be modifiedin various ways. For example, more (or less) groups of beam foldingmirrors can be used in each laser scanning station within the systemand/or more or less facets can be used for the rotating polygonalmirrors. Such modifications will add (or remove) scanning planes fromthe system.

Also more or less laser scanning stations might be employed within thesystem. Such modifications might be practiced in order to provide anomni-directional laser scanning pattern having scanning performancecharacteristics optimized for a specialized scanning application.

While the second rotating polygonal mirror of the illustrativeembodiment employs facets having low and high elevation anglecharacteristics, it is understood that it might be desirable inparticular applications to use scanning facets with differentcharacteristics (such as varying angular reflection characteristics) soas to enable a compact scanner design in a particular application.

Also, it is contemplated that each laser scanning station may not haveits own laser source (e.g., VLD). More specifically, as is well known inthe scanning art, the laser light produced by a laser source (VLD) maybe split into multiple beams (with a beam splitter) and directed tomultiple laser scanning stations with mirrors, a light pipe or otherlight directing optical element.

Also, it is contemplated that the logical assignment of laser scanningstations (or laser scanning planes or laser scanning plane groups) toTDM time slots can be varied in accordance with the present invention.

TDM Timing Signal Generation

FIGS. 8A though 10 depict various aspects of mechanisms for performingtime-division-multiplexed laser scanning operations and signalprocessing operations. Such mechanisms may be embodied in the abovedescribed embodiments of laser scanning systems in accordance with thepresent invention.

FIG. 8A is a functional block diagram of electronic circuitry thatderives timing signals that synchronize the time-division multiplexedlaser scanning and signal processing operations performed by a laserscanning system in accordance with the present invention. For example,the electronic circuitry of FIG. 8A may be integrated as part of the VLDcontrol module of the illustrative bioptical laser scanning system asshown in FIG. 7. FIGS. 8B1 through 8B9 illustrate timing signalsgenerated by the circuit elements of FIG. 8A.

In the illustrative embodiment of the present invention shown FIGS. 8Aand 8B1 through 8B9, each time-division-multiplexed laser scanning beamscans the surface of articles disposed in the scanning volume at asampling frequency that is at least two times the highest frequencycomponent expected in the scan data signal, in accordance with the wellknown Nyquist criteria. It is well established that the highestfrequency component expected in the scan data signal (sometimes referredto as “Nyquist frequency”) can be approximated with a simplified modelby dividing the maximum beam velocity over the width of minimum bar codewidth, where the maximum beam velocity is dependent on the maximum depthof field of the laser scanner, the angular velocity of the rotatingpolygonal mirror and the scan factor of the scanning mechanism (whichdefines the relationship between the angular velocity of the scanningbeam and the angular velocity of the rotating polygonal mirror).

In the illustrative laser scanning mechanism described above, theNyquist frequency of the expected scan data signal lies at approximately1 MHz. In this scenario, each time-division-multiplexed laser scanningbeam (from HST1, HST2 and VST1) and corresponding signal processingcircuitry may operate at a frequency of approximately 5 MHz, which isgreater than four times the Nyquist frequency. By operating the TDMlaser scanning and signal processing (bar code detection) mechanisms ata high frequency (e.g., at a frequency greater than at least two timesthe Nyquist frequency of the laser scanning system, and preferably at asampling frequency much greater than two times the Nyquist frequency ofthe laser scanning system, such as at a sampling frequency four to sixtimes the Nyquist frequency of the laser scanning system), the TDM laserscanning and signal processing (bar code detection) of each scan lineprovides adequate scan resolution for reading high density bar codesymbols disposed in the furthest focal zones of the system, whileproviding improved bandwidth (i.e., rate at which information isacquired by the laser scanning mechanism) as compared to other TDM-basedlaser scanning and signal processing operations that utilize a lowerfrequency and comparable mirror speeds.

Alternatively, each time-division-multiplexed laser scanning beam (fromHST1, HST2 and VST1) and corresponding signal processing circuitry mayoperate at a lower frequency (e.g., at a frequency less than the 1 MHzNyquist frequency, such as at a frequency corresponding to scan lines orscan line groups as described above). By operating the TDM laserscanning and signal processing (bar code detection) mechanisms at alower frequency, cross-talk is avoided; yet the bandwidth (i.e., rate atwhich information is acquired by the laser scanning mechanism) of thescanning system is reduced as compared to other TDM-based laser scanningand signal processing operations that utilize a higher frequency andcomparable mirror speeds.

The circuitry of FIG. 8A includes an oscillator and phase-lock loopsynthesizer that cooperate to generate a 15 MHz clock signal as shown inFIG. 8B 1. The 15 MHz clock signal is supplied to a 3-state counter,which cycles through its 3 states in response to positive pulses in the15 MHz clock signal as shown in FIG. 8B 2. The output of the 3-statecounter, which identifies its current state (state1, state2, or state3),is supplied to state 1 analysis logic, state 2 analysis logic and state3 analysis logic. The state 1 analysis logic outputs a firstpredetermined logic level signal (e.g., H logic level) in the event thatthe output of the 3-state counter identifies its current state as state1, and otherwise outputs a second predetermined logic level (e.g., Llogic level) as shown in FIG. 8B 3. The state 2 analysis logic outputs afirst predetermined logic level signal (e.g., H logic level) in theevent that the output of the 3-state counter identifies its currentstate as state 2, and otherwise outputs a second predetermined logiclevel (e.g., L logic level) as shown in FIG. 8B 4. The state 3 analysislogic outputs a first predetermined logic level signal (e.g., H logiclevel) in the event that the output of the 3-state counter identifiesits current state as state 3, and otherwise outputs a secondpredetermined logic level (e.g., L logic level) as shown in 18B5. Theoutput of the state 1 analysis logic and the 15 MHz signal is suppliedto a first AND gate (AND1), which generates the HST1_ENABLE Signal asshown in FIG. 8B 6, which is a pulse train whose positive pulses definetime slots that are logically assigned to a unique laser light sourceand photosensor (for example, VLD_(HST1) and PD_(HST1) of the firstlaser scanning station HST1 in the illustrative bioptical laser scanningsystem described above). The output of the state 2 analysis logic andthe 15 MHz signal is supplied to a second AND gate (AND2), whichgenerates the HST2_ENABLE Signal as shown in FIG. 8B 7, which is a pulsetrain whose positive pulses define time slots that are logicallyassigned to a unique laser light source and photosensor (for example,VLD_(HST2) and PD_(HST2) of the second laser scanning station HST2 inthe illustrative bioptical laser scanning system described above). Theoutput of the state 3 analysis logic and the 15 MHz signal is suppliedto a third AND gate (AND3), which generates the VST1_ENABLE Signal asshown in FIG. 8B 8, which is a pulse train whose positive pulses definetime slots that are logically assigned to a unique laser light sourceand photosensor (for example, VLD_(VST1) and PD_(VST1) of the thirdlaser scanning station VST1 in the illustrative bioptical laser scanningsystem described above).

During the time slots defined by the timing signals HST1_ENABLE,HST2_ENABLE and VST1_ENABLE, the laser scanning system performstime-division multiplexed laser scanning operations whereby, during eachgiven time slot, the output optical power of the laser light sources ofthe system (e.g., HST1 VLD, HST2 VLD and VST1 VLD) are modulated suchthat the laser light source logically assigned thereto is turned ON andprojected into the scanning volume while the other laser light sources(those not logically assigned to the given time slot) are turnedsubstantially OFF. Alternatively, instead of modulating the power ofmultiple laser light sources, a shared laser light source may cooperatewith an optical shutter or beam steering device to produce thetime-division-multiplexed laser scanning beams as illustrated in FIG.10. In addition, the signal processing circuitry that performs bar codedetection and operations on the data signals derived from thephotosensors of the system (e.g., PD_(HST1), PD_(HST2), and PD_(VST1))operates synchronously in response to these timing signals such that,during each given time slot (or portion of the given time slot), thephotosensor logically assigned thereto is operably coupled to the signalprocessing circuitry that performs bar code detection operations on thedata signals derived therefrom while the other photosensors (those notlogically assigned to the given time slot) are operably decoupled (e.g.,electrically isolated) from such signal processing circuitry.

FIG. 8C is a table that describes time-division multiplexed laserscanning and signal processing operations performed by the illustrativebioptical laser scanning system described herein in accordance with thepresent invention; such time-division multiplexed laser scanning andsignal processing operations are performed during time slots defined bythe timing signals of FIGS. 8B1 through 8B9.

As summarized in the table of FIG. 8C, the following operations areperformed during the “HST1” time slots when the HST1_Enable Signal isactive:

-   -   i) during the “HST1” time slots, generation and/or projection of        the scanning laser beam from HST1 into the scanning volume is        enabled; and    -   ii) during a portion of the “HST1” time slots, photosensor        PD_(HST1) is operably coupled to signal processing circuitry        that performs bar code detection and decoding operations on data        signals derived from the output of photosensor PD_(HST1).

Note that during the “HST1” time slots, the HST2_Enable Signal and theVST1_Enable Signal are inactive. As summarized in the table of FIG. 1D,the following operations are performed during the “HST1” time slots whenthe HST2_Enable Signal and VST1_Enable Signal are inactive:

-   -   i) generation and/or projection of the scanning laser beam from        HST2 into the scanning volume is disabled;    -   ii) photosensor PD_(HST2) is operably decoupled (e.g.,        electrically isolated) from bar code detection signal processing        circuitry.    -   i) generation and/or projection of the scanning laser beam from        VST1 into the scanning volume is disabled; and    -   ii) photosensor PD_(VST1) is operably decoupled (e.g.,        electrically isolated) from bar code detection signal processing        circuitry.

In the event that the time-division-multiplexed laser scanningoperations of the scanning laser beams from HST2 and VST1 andcorresponding signal processing operations performed on the output ofphotosensors PD_(HST2) and PD_(VST1) are performed at a high frequency(e.g., at a frequency greater than two times the Nyquist frequency andpossibly greater than four times the Nyquist frequency as describedherein), which is determined by the frequency of HST2_Enable Signal andVST1_Enable Signal, bar code detection signal processing circuitryoperates on a sample of the data signals derived from the photosensorsPD_(HST2) and PD_(VST1) during the previous “HST2” time slot and “VST1”time slot, respectively. Such operations are preferably provided bytrack and hold circuitry (or sample and hold circuitry which is wellknown in the electronic arts) which operably couples/decouples (e.g.,electrically isolates) the output of a photosensors PD_(HST2) andPD_(VST1) from bar code detection signal processing circuitry, andstores a sample of the data signals derived from the photosensorsPD_(HST2) and PD_(VST1) during the previous “HST2” and “VST1” timeslots, respectively, on hold capacitors that are operably coupled to barcode detection signal processing circuitry when the HST2_Enable Signaland VST1_Enable Signal are inactive (which is during the “HST1” timeslots).

In the event that the time-division-multiplexed laser scanningoperations of the scanning laser beams from HST2 and VST1 andcorresponding signal processing operations performed on the output ofphotosensors PD_(HST2) and PD_(VST1) are performed at a lower frequency(e.g., at a frequency much less than the Nyquist frequency as describedherein), which is determined by the frequency of HST2_Enable Signal andVST1_Enable Signal, bar code detection signal processing circuitry doesnot operate on samples of data signals derived from the photosensorsPD_(HST2) and PD_(VST1) during the “HST1” time slots. It should be notedthat in such embodiments, the bar code detection signal processingcircuitry operates at frequencies at least two times greater than theNyquist frequency in sampling and processing the data signals derivedfrom the photosensor PD_(HST1) and supplied thereto during the “HST1”time slots in order to detect and decode bar code symbols therein.

Similar time-division multiplexed scanning and signal processingoperations are performed by HST2 and the output of PD_(HST2) during the“slot_2” time slots. More specifically, as summarized in the table ofFIG. 8C, the following operations are performed during the “HST2” timeslots when the HST2_Enable Signal is active:

-   -   i) during the “HST2” time slots, generation and/or projection of        the scanning laser beam from HST2 into the scanning volume is        enabled; and    -   ii) during a portion of the “HST2” time slots, photosensor        PD_(HST2) is operably coupled to signal processing circuitry        that performs bar code detection and decoding operations on data        signals derived from the output of photosensor PD_(HST2).

Note that during the “HST2” time slots, the HST1_Enable Signal and theVST1_Enable Signal are inactive. As summarized in the table of FIG. 8C,the following operations are performed during the “HST2” time slots whenthe HST1_Enable Signal and VST1_Enable Signal are inactive:

-   -   i) generation and/or projection of the scanning laser beam from        HST1 into the scanning volume is disabled;    -   ii) photosensor PD_(HST1) is operably decoupled (e.g.,        electrically isolated) from bar code detection signal processing        circuitry.    -   i) generation and/or projection of the scanning laser beam from        VST1 into the scanning volume is disabled; and    -   ii) photosensor PD_(VST1) is operably decoupled (e.g.,        electrically isolated) from bar code detection signal processing        circuitry.

In the event that the time-division-multiplexed laser scanningoperations of the scanning laser beams from HST1 and VST1 andcorresponding signal processing operations performed on the output ofphotosensors PD_(HST1) and PD_(VST1) are carried out at a high frequency(e.g., at a frequency greater than two times the Nyquist frequency andpossibly greater than four times the Nyquist frequency as describedherein), which is determined by the frequency of HST1_Enable Signal andVST1_Enable Signal, bar code detection signal processing circuitryoperates on a sample of the data signals derived from the photosensorsPD_(HST1) and PD_(VST1) during the previous “HST1” time slot and “VST1”time slot, respectively. Such operations are preferably provided bytrack and hold circuitry (or sample and hold circuitry which is wellknown in the electronic arts) which operably couples/decouples (e.g.,electrically isolates) the output of a photosensors PD_(HST1) andPD_(VST1) from bar code detection signal processing circuitry, andstores a sample of the data signals derived from the photosensorsPD_(HST1) and PD_(VST1) during the previous “HST1” and “VST1” timeslots, respectively, on hold capacitors that are operably coupled to barcode detection signal processing circuitry when the HST1_Enable Signaland VST1_Enable Signal are inactive (which is during the “HST1” timeslots).

In the event that the time-division-multiplexed laser scanningoperations of the scanning laser beams from HST1 and VST1 andcorresponding signal processing operations performed on the output ofphotosensors PD_(HST1) and PD_(VST1) are carried out at a lowerfrequency (e.g., at a frequency much less than the Nyquist frequency asdescribed herein), which is determined by the frequency of HST1_EnableSignal and VST1_Enable Signal, bar code detection signal processingcircuitry does not operate on samples of data signals derived from thephotosensors PD_(HST1) and PD_(VST1) during the “HST2” time slots. Itshould be noted that in such embodiments, the bar code detection signalprocessing circuitry operates at frequencies at least two times greaterthan the Nyquist frequency in sampling and processing the data signalsderived from the photosensor PD_(HST2) and supplied thereto during the“HST2” time slots in order to detect and decode bar code symbolstherein.

Similar time-division multiplexed scanning and signal processingoperations are performed by VST1 and the output of PD_(VST1) during the“VST1” time slots. More specifically, as summarized in the table of FIG.8C, the following operations are performed during the “VST1” time slotswhen the VST1_Enable Signal is active:

-   -   i) during the “VST1” time slots, generation and/or projection of        the scanning laser beam from VST1 into the scanning volume is        enabled; and    -   ii) during a portion of the “VST1” time slots, photosensor        PD_(VST1) is operably coupled to signal processing circuitry        that performs bar code detection and decoding operations on data        signals derived from the output of photosensor PD_(VST1).

Note that during the “VST1” time slots, the HST1_Enable Signal and theHST2_Enable Signal are inactive. As summarized in the table of FIG. 8C,the following operations are performed during the “VST1” time slots whenthe HST1_Enable Signal and HST2_Enable Signal are inactive:

-   -   i) generation and/or projection of the scanning laser beam from        HST1 into the scanning volume is disabled;    -   ii) photosensor PD_(HST1) is operably decoupled (e.g.,        electrically isolated) from bar code detection signal processing        circuitry.    -   i) generation and/or projection of the scanning laser beam from        HST2 into the scanning volume is disabled; and    -   ii) photosensor PD_(HST2) is operably decoupled (e.g.,        electrically isolated) from bar code detection signal processing        circuitry.

In the event that the time-division-multiplexed laser scanningoperations of the scanning laser beams from HST1 and HST2 andcorresponding signal processing operations performed on the output ofphotosensors PD_(HST1) and PD_(HST2) are carried out at a high frequency(e.g., at a frequency greater than two times the Nyquist frequency andpossibly greater than four times the Nyquist frequency as describedherein), which is determined by the frequency of HST1_Enable Signal andHST2_Enable Signal, bar code detection signal processing circuitryoperates on a sample of the data signals derived from the photosensorsPD_(HST1) and PD_(HST2) during the previous “HST1” time slot and “HST2”time slot, respectively. Such operations are preferably provided bytrack and hold circuitry (or sample and hold circuitry which is wellknown in the electronic arts) which operably couples/decouples (e.g.,electrically isolates) the output of a photosensors PD_(HST1) andPD_(HST2) from bar code detection signal processing circuitry, andstores a sample of the data signals derived from the photosensorsPD_(HST1) and PD_(HST2) during the previous “HST1” and “HST2 time slots,respectively, on hold capacitors that are operably coupled to bar codedetection signal processing circuitry when the HST1_Enable Signal andHST2_Enable Signal are inactive (which is during the “VST1” time slots).

In the event that the time-division-multiplexed laser scanningoperations of the scanning laser beams from HST1 and HST2 andcorresponding signal processing operations performed on the output ofphotosensors PD_(HST1) and PD_(HST2) are carried out a lower frequency(e.g., at a frequency much less than the Nyquist frequency as describedherein), which is determined by the frequency of HST1_Enable Signal andHST2_Enable Signal, bar code detection signal processing circuitry doesnot operate on samples of data signals derived from the photosensorsPD_(HST1) and PD_(HST2) during the “VST1” time slots. It should be notedthat in such embodiments, the bar code detection signal processingcircuitry operates at frequencies at least two times greater than theNyquist frequency in sampling and processing the data signals derivedfrom the photosensor PD_(VST1) and supplied thereto during the “VST1”time slots in order to detect and decode bar code symbols therein.

TDM Laser Scanning Operations

FIG. 8D is a high-level functional block diagram of electronic circuitrythat performs time-division multiplexed laser scanning operations inresponse to the timing signals of FIGS. 8B6 through 8B8 in accordancewith the present invention. For example, the electronic circuitry ofFIG. 8D may be integrated as part of the VLD control module andmodulation enabled VLD drive circuitry of the illustrative biopticallaser scanning system as shown in FIG. 7. The HST1 modulation enableddrive circuitry is operably coupled to a first VLD (HST1 VLD) andoperates in response to the HST1_ENABLE signal to modulate the outputoptical power of the HST1 VLD such that: the HST1 VLD is turned ON andprojected into the scanning volume during the time slots logicallyassigned to the HST1 VLD (as encoded by the pulses of the HST1_ENABLEsignal) while the HST1 VLD is turned substantially OFF during timeperiods outside the time slots logically assigned to the HST1 VLD.Similarly, the HST2 modulation enabled drive circuitry is operablycoupled to a second VLD (HST2 VLD) and operates in response to theHST2_ENABLE signal to modulate the output optical power of the HST2 VLDsuch that: the HST2 VLD is turned ON and projected into the scanningvolume during the time slots logically assigned to the HST2 VLD (asencoded by the pulses of the HST2_ENABLE signal) while the HST2 VLD isturned substantially OFF during time periods outside the time slotslogically assigned to the HST2 VLD. And the VST1 modulation enableddrive circuitry is operably coupled to a third VLD (VST1 VLD) andoperates in response to the VST1_ENABLE signal to modulate the outputoptical power of the VST1 VLD such that: the VST1 VLD is turned ON andprojected into the scanning volume during the time slots logicallyassigned to the VST1 VLD (as encoded by the pulses of the VST1_ENABLEsignal) while the VST1 VLD is turned substantially OFF during timeperiods outside the time slots logically assigned to the VST1 VLD.

FIGS. 8E1-8G2 depict mechanisms for modulating the optical power ofmultiple VLD modules to provide multiple time-division-multiplexed laserscanning beams in accordance with the present invention.

As shown in FIG. 8E 1, a monitor photodiode is provided for each laserscanning station HST1, HST2, VST1 (preferably as an integral part of thecorresponding VLD module as shown), and each VLD Drive Circuit (HST1Drive Circuitry, HST2 Drive Circuitry and VST1 Drive Circuitry) includesautomatic power control circuitry, a dynamic current source andmodulation control circuitry. Generally, when the Enable Timing Signalfor a given VLD Drive Circuit is ON (e.g., Logic Level=H), feedback fromthe monitor photodiode of the given VLD Drive Circuit is input toautomatic power control circuitry operably coupled thereto—whichcontrols the dynamic current source of the given VLD Drive Circuit tomaintain the output optical power of the visible laser diode operablycoupled thereto at a predetermined power level P_(ON) to compensate forthermal drift and other forms of drift over the operating lifetime ofthe visible laser diode. The output power level P_(ON) is substantiallygreater than zero, e.g., on the order of 1 to 5 mW for typical VLDs usedin bar code reading applications. Yet, when the Enable Timing Signal fora given VLD Drive Circuit is OFF (e.g., Logic Level=L), the modulationcontrol circuit of the given VLD Drive Circuit controls the dynamiccurrent source to maintain the output optical power of the visible laserdiode operably coupled thereto at a predetermined level P_(OFF), whichis substantially less than P_(ON) (and preferably at or near zero). Thetable of FIG. 8E 2 summarizes these operations for the HST1 VLD DriveCircuitry. Analogous operations are carried out by the HST2 VLD DriveCircuitry and the VST1 VLD Drive Circuitry in order to provide multipletime-division-multiplexed laser scanning beams in accordance with thepresent invention.

The graphical representation of FIG. 8E 3 illustrates the optical powercharacteristics output from an exemplary visible laser diode in responseto current supplied thereto. In addition the graphical representationshows the output power levels P_(ON) and P_(OFF) of the exemplaryvisible laser diode in response to the logic levels H and L,respectively, of the enable timing signal (e.g., HST1_ENABLE) during thetime-division multiplexed laser scanning operations of FIG. 8E 2.

FIG. 8F 1 is a functional block diagram of an illustrative embodiment ofHST1 VLD Drive Circuitry and HST1 VLD Module of FIG. 8E 1 in accordancewith the present invention. Such circuitry carries out time-divisionmultiplexed laser scanning operations for HST1 in response to the timingsignals of FIGS. 8B6 through 8B8 accordance with the table of FIG. 8E 2and graph of FIG. 8E 3. Analogous circuitry may be embodied as part ofthe HST2 Drive Circuitry and HST2 VLD Module as well as the VST1 DriveCircuitry and VST1 VLD Module, which carries out time-divisionmultiplexed laser scanning operations for HST2 and VST1, respectively,in response to the timing signals of FIGS. 8B6 through 8B8.

The HST1 VLD Drive Circuitry of FIG. 8F 1 operates the visible laserdiode in the “OFF” state (by controlling a dynamic current source)whereby the output optical power level range (P_(OFF)) of the visiblelaser diode is significantly less than the threshold output opticalpower level (P_(TH)) of the visible laser diode.

FIG. 8F 2 is a graphical representation of the optical powercharacteristics output from an exemplary visible laser diode (which ispart of the HST1 VLD Module shown in FIG. 8F 1) in response to currentsupplied thereto. In addition, the graph shows the output power levelsP_(ON) and P_(OFF) of the visible laser diode in response to the logiclevels H and L, respectively, of the corresponding HST1_ENABLE signalduring time-division multiplexed laser scanning operations. Note that inthe illustrative embodiment of FIG. 8F 1 and 8F2, the current levelI_(Laser) that is supplied to the visible laser diode is near zero(I_(Laser)=I_(MOD)=I_(off)≈0) in response to the “L” logic level of theenable timing signal in order to produce the P_(OFF) output power level(e.g., near zero output power level). In contrast, the current levelI_(Laser) that is supplied to the visible laser diode is substantiallygreater than the threshold current I_(TH)(I_(Laser)=I_(MOD)=I_(ON)>>I_(TH)) in response to the “H” logic level ofthe enable timing signal in order to produce the P_(ON) output powerlevel (which is substantially greater than near zero, e.g., on the orderof 1 to 5 mW for typical visible laser diodes utilized in bar codereading applications).

In the illustrative circuit embodiment of FIG. 8F 1, a dynamic currentsource (realized by transistor Q1, Op-Amp O and resistor R_(MOD))provides the drive current I_(Laser) that is adjusted by the automaticpower control circuitry (which may be a difference amplifier thatamplifies the difference between voltage levels provided by the monitorphotodiode/resistor R_(MON) and a reference as shown) so that theoptical power of the visible laser diode is maintained at apredetermined level P_(ON) in the ON State, which compensates forvariations in laser current over temperature and lifetime of the system.The modulation control circuit (which may be a FET transistor as shown)activates the dynamic current source in response to the respectivemodulation control signal (HST1_ENABLE SIGNAL, HST2_ENABLE SIGNAL,VST1_ENABLE SIGNAL) being turned “ON” (in the H Logic Level State).

FIG. 8G 1 is a functional block diagram of an alternate embodiment ofthe HST1 VLD Drive Circuitry and HST1 VLD Module of FIG. 8E 1 inaccordance with the present invention. Such circuitry carries outtime-division multiplexed laser scanning operations for HST1 in responseto the timing signals of FIGS. 8B6 through 8B8, in accordance with thetable of FIG. 8E 2. Analogous circuitry may be embodied as part of theHST2 Drive Circuitry and HST2 VLD Module as well as the VST1 DriveCircuitry and VST1 VLD Module, which carries out time-divisionmultiplexed laser scanning operations for HST2 and VST1, respectively,in response to the timing signals of FIGS. 8B6 through 8B8.

The HST1 VLD Drive Circuitry of FIG. 8G 1 operates the visible laserdiode in the “OFF” state (by controlling a dynamic current source)whereby the output optical power level range (P_(OFF)) of the visiblelaser diode is near the threshold output optical power level (P_(TH)) ofthe visible laser diode. This second technique is preferably in manyapplications because it avoids delays in bringing the output opticalpower of the visible laser diode from the OFF state to the ON state inresponse to the respective modulation control signal (HST1_ENABLESIGNAL, HST2_ENABLE SIGNAL, VST1_ENABLE SIGNAL).

FIG. 8G 2 is a graphical representation of the optical powercharacteristics output from an exemplary visible laser diode (which ispart of the HST1 VLD Module shown in FIG. 8G 1) in response to currentsupplied thereto. In addition, the graph shows the output power levelsP_(ON) and P_(OFF) of the visible laser diode in response to the logiclevels H and L, respectively, of the corresponding HST1_ENABLE signalduring time-division multiplexed laser scanning operations. Note that inthe illustrative embodiment of FIG. 8G 1 and 8G2, the current levelI_(Laser) that is supplied to the visible laser diode is near thethreshold current I_(TH) (I_(Laser)=I_(BIAS)=I_(Off)≈I_(TH)) in responseto the “L” logic level of the HST1_Enable timing signal in order toproduce the P_(OFF) output power level (e.g., near zero output powerlevel). In contrast, the current level I_(Laser) that is supplied to thevisible laser diode is substantially greater than the threshold currentI_(TH) (I_(Laser)=(I_(BIAS)+I_(MOD))=I_(ON)>>I_(TH)) in response to the“H” logic level of the HST1_Enable timing signal in order to produce theP_(ON) output power level (which is substantially greater than nearzero, e.g., on the order of 1 to 5 mW for typical visible laser diodesused in bar code reading applications).

In the illustrative circuit embodiment of FIG. 8G 1, a dynamic currentsource (realized by resistor R_(BIAS), transistor Q1, Op-Amp O andresistor R_(MOD)) provides the drive current I_(Laser) that is adjustedby the automatic power control circuitry (which may be a differenceamplifier that amplifies the difference between voltage levels providedby the monitor photodiode/resistor R_(MON) and a reference as shown) sothat the optical power of the visible laser diode is maintained at apredetermined level P_(ON) in the ON State, which compensates forvariations in laser current over temperature and lifetime of the system.The resistor R_(BIAS) provides static current that is at (or near) thethreshold current I_(TH) of the visible laser diode. The modulationcontrol circuit (which may be a FET transistor as shown) activates thedynamic current source in response to the respective modulation controlsignal (HST1_ENABLE SIGNAL, HST2_ENABLE SIGNAL, VST1_ENABLE SIGNAL)being turned “ON” (in the H Logic Level State). In this configuration,The HST1 VLD Drive Circuitry operates the visible laser diode in the“OFF” state (by providing a static current I_(TH)) such that the outputoptical power level range (P_(OFF)) of the visible laser diode is nearits threshold output optical power level (P_(TH)), which avoids delaysin bringing the output optical power of the visible laser diode from theOFF state to the ON state in response to the respective modulationcontrol signal (HST1_ENABLE SIGNAL, HST2_ENABLE SIGNAL, VST1_ENABLESIGNAL).

Advantageously, such time-division multiplexed laser scanning operationsenable the laser scanning system of the present invention to scan thescanning volume with a plurality of multi-dimensional laser scanningbeams to detect and decode bar code symbols on surfaces disposed thereinwhile avoiding optical crosstalk that might otherwise occur and degradeperformance of the laser scanning system.

TDM Signal Processing Operations

FIG. 9A is a functional block diagram of exemplary electronic circuitrythat performs time-division multiplexed signal processing operations inresponse to the timing signals of FIGS. 8B6 through 8B8, which issuitable for use in the illustrative bioptical laser scanning systemwhose system architecture is shown in FIG. 7. In this illustrativeembodiment, time-division-multiplexed laser scanning operations andcorresponding signal processing operations are carried out at a highfrequency (e.g., at a frequency greater than two times the Nyquistfrequency and possibly greater than four times the Nyquist frequency asdescribed herein). As shown, track and hold circuitry operates inresponse to the timing signals of FIGS. 8B6 through 8B8 to operablycouple/de-couple the photosensors PD_(HST1), PD_(HST2) and PD_(VST1)to/from signal processing circuitry that performs bar code detection anddecoding on data signals derived from the output of the photosensorsPD_(HST1), PD_(HST2) and PD_(VST1) during the time slots (or a portionof the time slots) assigned to photosensor PD_(HST1), PD_(HST2) orPD_(VST1) (and laser scanning station HST1, HST2 or VST1) correspondingthereto.

More specifically, during the time slots assigned to photosensorPD_(HST1) (or a portion of the time slots assigned to photosensorPD_(HST1)), the switch (switch_HST1) of track and hold circuit 1 isclosed to operably couple photosensor PD_(HST1) to the signal processingcircuitry that performs bar code detection and decoding on data signalsderived from the output of the photosensor PD_(HST1). With switch_HST1closed (i.e., “track mode), the voltage levels at the output of trackand hold circuit 1 track changes in the voltage levels at its input(which correspond to the output of photosensor PD_(HST1)). The output ofthe track and hold circuit 1 is preferably supplied to filter circuitry(which filters out unwanted noise components) and the resultant signalis supplied to bar code detection signal processing circuitry. Outsidethe time slots (or time slot portions) assigned to photosensorPD_(HST1), switch_HST1 remains open to operably decouple (e.g.,electrically isolate) photosensor PD_(HST1) from the filter circuitryand signal processing circuitry. With switch_HST1 open (i.e., “holdmode”), the hold capacitor C_hold1 stores a sample of the data signalsderived from the photosensor PD_(HST1) during the previous time slotassigned to PD_(HST1), and the voltage level at the output of track andhold circuit 1 corresponds to the sampled voltage level stored on thehold capacitor C_hold1. The output of the track and hold circuit 1 issupplied to filter circuitry (which filters out unwanted noisecomponents) and the resultant signal is supplied to bar code detectionsignal processing circuitry. In this manner, during the “hold mode”, barcode detection signal processing circuitry operates on a sample of thedata signals derived from the photosensor PD_(HST1) during the previoustime slot assigned thereto.

Similarly, during the time slots assigned to photosensor PD_(HST2) (or aportion of the time slots assigned to photosensor PD_(HST2)), the switch(switch_HST2) of track and hold circuit 2 is closed to operably couplephotosensor PD_(HST2) to the signal processing circuitry that performsbar code detection and decoding on data signals derived from the outputof the photosensor PD_(HST2). With switch_HST2 closed (i.e., “track”mode), the voltage levels at the output of track and hold circuit 2track changes in the voltage levels at its input (which correspond tothe output of photosensor PD_(HST2)). The output of the track and holdcircuit 2 is preferably supplied to filter circuitry (which filters outunwanted noise components) and the resultant signal is supplied to barcode detection signal processing circuitry. Outside the time slots (ortime slot portions) assigned to photosensor PD_(HST2), switch_HST2remains open to operably decouple (e.g., electrically isolate)photosensor PD_(HST2) from the filter circuitry and signal processingcircuitry. With switch_HST2 open (i.e., “hold mode”), the hold capacitorC_hold2 stores a sample of the data signals derived from the photosensorPD_(HST2) during the previous time slot assigned to PD_(HST2), and thevoltage level at the output of track and hold circuit 2 corresponds tothe sampled voltage level stored on the hold capacitor C_hold2. Theoutput of the track and hold circuit 2 is supplied to filter circuitry(which filters out unwanted noise components) and the resultant signalis supplied to bar code detection signal processing circuitry. In thismanner, during the “hold mode”, bar code detection signal processingcircuitry operates on a sample of the data signals derived from thephotosensor PD_(HST2) during the previous time slot assigned thereto.

Similarly, during the time slots assigned to photosensor PD_(VST1) (or aportion of the time slots assigned to photosensor PD_(VST1)), the switch(switch_VST1) of track and hold circuit 3 is closed to operably couplephotosensor PD_(VST1) to the signal processing circuitry that performsbar code detection and decoding on data signals derived from the outputof the photosensor PD_(VST1). With switch_VST1 closed (i.e., “trackmode), the voltage levels at the output of track and hold circuit 3track changes in the voltage levels at its input (which correspond tothe output of photosensor PD_(VST1)). The output of the track and holdcircuit 3 is preferably supplied to filter circuitry (which filters outunwanted noise components) and the resultant signal is supplied to barcode detection signal processing circuitry. Outside the time slots (ortime slot portions) assigned to photosensor PD_(VST1), switch_VST1remains open to operably decouple (e.g., electrically isolate)photosensor PD_(VST1) from the filter circuitry and signal processingcircuitry. With switch_VST1 open (i.e., “hold mode”), the hold capacitorC_hold3 stores a sample of the data signals derived from the photosensorPD_(VST1) during the previous time slot assigned to PD_(VST1), and thevoltage level at the output of track and hold circuit 3 corresponds tothe sampled voltage level stored on the hold capacitor C_hold3. Theoutput of the track and hold circuit 3 is supplied to filter circuitry(which filters out unwanted noise components) and the resultant signalis supplied to bar code detection signal processing circuitry. In thismanner, during the “hold mode”, bar code detection signal processingcircuitry operates on a sample of the data signals derived from thephotosensor PD_(VST1) during the previous time slot assigned thereto.

FIG. 9B is a circuit diagram of an exemplary track and hold circuitsuitable for use in the time-division multiplexed signal processingcircuitry of FIG. 9A. The track and hold circuit includes an inputbuffer (provided by a 0.01 uF capacitor and 5 Kohm resistor for ACcoupling, an op-amp configured as a voltage follower, and 10 K ohmresistor coupled between the output node of the op-amp and ground), aFET switch whose source/drain is coupled between the output node of theinput buffer and input node of an output buffer and whose gate iscoupled to a signal line that carries the SourceX_enable signal (toswitch between “track mode” and “hold mode”), a 470 pF hold capacitorcoupled between the input node of the output buffer and ground, and theoutput buffer (provided an op-amp configured as a non-invertingamplifier with a gain of 3 (e.g. 1+300/150). When the FET switch isclosed (i.e., “track mode), the voltage levels at the output of trackand hold circuit track changes in the voltage levels at its input. Yet,when the FET switch is open (i.e., “hold mode”), the hold capacitorstores a sample of the voltage levels at the input of the track and holdcircuit just prior to the FET switch being closed, and the voltage levelat the output of track and hold circuit corresponds to the sampledvoltage level stored on the hold capacitor.

In alternative embodiments, the TDM signal processing operationsperformed on the output of photosensors PD_(HST1), PD_(HST2), andPD_(VST1) may be carried out a lower frequency (e.g., at a frequencymuch less than the Nyquist frequency, such as over scan lines or scanline groups as described herein), which is determined by the frequencyof HST1_Enable Signal, HST2_Enable Signal and VST1_Enable Signal. Insuch embodiments, bar code detection signal processing circuitry doesnot operate on samples of data signals derived from the two photosensorsthat does not correspond to a given time slot; yet bar code detectionsignal processing circuitry does operate at frequencies at least twotimes greater than the Nyquist frequency in sampling and processing thedata signals derived from the photosensor corresponding to the giventime slot in order to detect and decode bar code symbols therein. Insuch embodiments, a multiplexing switch may be used (as a substitute fortrack and hold circuitry) to operably couple/decouple (e.g.,electrically isolate) the output of a given photosensor from bar codedetection signal processing circuitry, and there is no need to store(and process) a sample of the data signals derived from the givenphotosensor during the previous time slot logically assigned to thegiven photosensor.

FIG. 9C is a functional block diagram of exemplary electronic circuitrythat performs time-division multiplexed signal processing operations inresponse to the timing signals of FIGS. 8B6 through 8B8, which issuitable for use in the illustrative bioptical laser scanning systemdescribed above. In this illustrative embodiment, three analog todigital signal converters are provided, each processing data signalsderived from a different photosensor (PD_(HST1), PD_(HST2) orPD_(VST1)). The three analog to digital converters each sample theconditioned analog scan data signals at a sampling frequency at leasttwo times the highest frequency component expected in the analog scandata signal (in accordance with the well known Nyquist criteria) andquantizes each time-sampled scan data signal value into a discretesignal level using a suitable length number representation (e.g. 8 bits)to produce a discrete scan data signal. One or more programmedprocessors (e.g., a digital signal processor and associated memory)perform digital signal processing functions on the discrete signallevels to generate a sequence of digital words (i.e., a sequence ofdigital count values), each representing the time length associated withthe signal level transitions in the corresponding digital scan datasignal as described above. Time-division multiplexed signal processingoperations are provided by selectively enabling the sampling function(and possibly the conversion function) of only one of the three analogto digital signal converters during the time slot assigned tophotosensor PD_(HST1), PD_(HST2) or PD_(VST1) (and laser scanningstation HST1, HST2 or VST1) corresponding thereto. More specifically,during the time slots assigned to photosensor PD_(HST1), the samplingfunction (and possibly the conversion function) of analog to digitalsignal converter A/D_(HST1) is selectively activated, and it is disabledduring time periods outside such time slots. Similarly, during the timeslots assigned to photosensor PD_(HST2), the sampling function (andpossibly the conversion function) of analog to digital signal converterA/D_(HST2) is selectively activated, and it is disabled during timeperiods outside such time slots. And during the time slots assigned tophotosensor PD_(VST1), the sampling function (and possibly theconversion function) of analog to digital signal converter A/D_(VST1) isselectively activated, and it is disabled during time periods outsidesuch time slots.

FIG. 9D is a functional block diagram of exemplary electronic circuitrythat performs time-division multiplexed signal processing operations inresponse to the timing signals of FIGS. 8B6 through 8B8, which issuitable for use in the illustrative bioptical laser scanning systemdescribed above. In this illustrative embodiment, a shared analog todigital signal converter is provided that processes data signals derivedfrom any one of the three photosensors PD_(HST1), PD_(HST2) andPD_(VST1). Multiplexing switches operate in response to the timingsignals of FIGS. 8B6 through 8B8 to operably couple/de-couple a givenphotosensor (PD_(HST1), PD_(HST2) or PD_(VST1)) to the shared analog todigital signal converter during each time slot assigned to the givenphotosensor (PD_(HST1), PD_(HST2) or PD_(VST1) (and laser scanningstation HST1, HST2 or VST1). The shared analog to digital converterssamples the conditioned analog scan data signals at a sampling frequencyat least two times the highest frequency component expected in theanalog scan data signal (in accordance with the well known Nyquistcriteria) and quantizes each time-sampled scan data signal value into adiscrete signal level using a suitable length number representation(e.g. 8 bits) to produce a discrete scan data signal. One or moreprogrammed processors (e.g., a digital signal processor and associatedmemory) perform digital signal processing functions on the discretesignal levels to generate a sequence of digital words (i.e., a sequenceof digital count values), each representing the time length associatedwith the signal level transitions in the corresponding digital scan datasignal as described above. The time-division multiplexed signalprocessing operations are provided by selectively enabling the sharedanalog to digital signal converter during the time slots assigned to thephotosensors PD_(HST1), PD_(HST2) and PD_(VST1) (and laser scanningstation HST1, HST2 and VST1) corresponding thereto. More specifically,during the time slots assigned to photosensor PD_(HST1), the switch(Switch_HST1) is closed to operably couple photosensor PD_(HST1) to theshared analog to digital converter, and remains open during time periodsoutside these time slots to operably decouple (e.g., electricallyisolate) photosensor PD_(HST1) from the shared analog to digitalconverter. Similarly, during the time slots assigned to photosensorPD_(HST2), the switch (Switch_HST2) is closed to operably couplephotosensor PD_(HST2) to the shared analog to digital converter, andremains open during time periods outside these time slots to operablydecouple (e.g., electrically isolate) photosensor PD_(HST2) from theshared analog to digital converter. And during the time slots assignedto photosensor PD_(VST1), the switch (Switch_VST1) is closed to operablycouple photosensor PD_(VST1) to the shared analog to digital converter,and remains open during time periods outside these time slots tooperably decouple (e.g., electrically isolate) photosensor PD_(VST1)from the shared analog to digital converter. Moreover, during the timeslots assigned to photosensors PD_(HST1), PD_(HST2) and PD_(VST1), theanalog to digital signal converter A/D_(HST2) is selectively activated,and it is disabled during time periods outside such time slots.

Advantageously, such time-division multiplexed signal processingoperations enable the laser scanning system of the present invention toscan the scanning volume with a plurality of multi-dimensional laserscanning beams to detect and decode bar code symbols on surfacesdisposed therein while avoiding light interference from ambient lightand unwanted reflections that might occur and degrade performance of thesystem.

Other Time-Division-Multiplexed Laser Scanning Mechanisms

FIG. 10 is a schematic representation of an alternate embodiment of thepresent invention wherein a time division multiplexed beam steeringdevice provides time-division multiplexed laser scanning beams (forexample, three shown) for scanning the scanning volume of the laserscanning system of the present invention. Such a device may embodied inthe illustrative bioptical laser scanning system described above toprovide three time-division multiplexed laser scanning beams to thethree laser scanning stations HST1, HST2 and VST1 over time slots (e.g.,time intervals) T1, T2 and T3 logically assigned thereto.

Multi-Path Scan Signal Processing

In any laser scanning system (including the various laser scannersystems described herein), the primary function of the laser scanningmechanism is to produce a laser scanning field (or volume) in which barcode symbols can be scanned in a reliable manner. In such systems, thespeed of the laser beam spot (or cross-section) along the extent of thescanned laser beam will vary over the depth of the scanning range of thesystem. The further the laser beam spot is away from the laser scanningmechanism, the greater the laser beam spot speed with be, based on wellknown principles of physics. A useful measure of such beam spot speedvariation is given by the ratio of (i) the maximum laser beam spot speedwithin the scanning field of the system, to (ii) the minimum laser beamspot speed in the scanning system. Hereinafter, this spot speedvariation measure shall be referred to as the “Max/Min Beam Spot SpeedRatio” of a laser scanning system.

The substrate, usually paper, on which a bar code is printed reflects asignal of varying power when scanned with a focused laser beam within agiven focal zone in the system. The laser light energy reflected (i.e.scattered) off the scanned code symbol is directed onto a photodetectorby way of light collection and focusing optics. The photodetectorconverts these optical signals into corresponding electrical signals.The signal components produced by scanning the bar code substrate areunwanted and therefore are described as noise. Since the substrate isusually paper, consisting of fibers having a random spatial structure,such unwanted noise signals are commonly referred to as paper orsubstrate noise. A signal derived from the output of the photodetector(in analog or digital form) is referred to as a scan data signalS_(analog) comprising the desired bar code signal component as well asthe paper noise components.

As a bar code is scanned within a focal zone disposed further away fromthe scanner, the scan data signal is increasingly compressed on thetime-domain by virtue of the fact that the laser beam speed increases asa function of distance away from the laser scanning mechanism. Inaccordance with Fourier Analysis principles, compression of the scandata signal (including its noise components) represented on thetime-domain results in an increase in or shift of power to the higherspectral components of the scan data signal represented on thefrequency-domain. Thus, the frequency spectra of the scan data signal(including its noise components) undergoes a positive frequency shift asthe corresponding bar code symbol is scanned further away from the laserscanning system. This phenomenon is graphically illustrated in theanalog scan data signal of FIGS. 11A and 11B.

When scanning bar code symbols in a multi-focal zone laser scanningsystem, filters and signal thresholding devices are useful for rejectingnoise components in the scan data signal. However, such devices alsolimit the scan resolution of the system, potentially rendering thesystem incapable of reading low contrast and high resolution bar codesymbols on surfaces placed in the scanning field. Thus, it is imperativethat the bandwidth of the system be sufficient to support the spectralcomponents of scan data signals at different focal zones of the systemand to support the scanning of the desired resolution of bar codesymbols on surfaces placed in the scanning field.

In accordance with teachings of the present invention, a laser scanningsystem (such as the illustrative bioptical laser scanning systemdescribed herein) includes a TDM-enabled multi-path scan data signalprocessor having multiple signal processing paths. Each signalprocessing path processes the same data signal (which is derived fromthe output of a photodetector) to detect bar code symbols therein andgenerate data representing the bar code symbols. And each signalprocessing path has different operational characteristics (such aslow-pass filter cutoff frequencies, amplifier gain characteristics,and/or positive and negative signal thresholds). The varying operationalcharacteristics of the paths are optimized to provide different signalprocessing functions (e.g., minimize paper noise, or maximize the scanresolution of the system). The data signal derived from laser scanningis supplied to each path of the multi-path scan data processor, where itis processed (preferably in parallel) to identify signal leveltransitions therein. A digital scan data signal that encodes such signallevel transitions is provided to digitizing circuitry, which convertsthe digital scan data signal into a corresponding sequence of digitalwords (i.e. a sequence of digital count values) suitable for bar codesymbol decoding as described above.

By virtue of this aspect of the present invention, it is now possible toidentify signal level transitions in the scan data signal over a diverserange of operating conditions (e.g., operating conditions where papernoise is present in addition to operating conditions requiring highresolution scanning, such as the reading of low contrast or highresolution bar code symbols), which enables more reliable bar codereading over such diverse operating conditions. These and otheradvantages of the present invention will become apparent hereinafter.

Analog Scan Data Signal Processor of the Illustrative Embodiment of thePresent Invention

As shown in FIG. 11C, a TDM-enabled multi-path scan data signalprocessor 901 according to the present invention comprises a number ofsubcomponents, namely: signal conditioning circuitry 903, track and holdcircuitry 911 and low pass filter 912 operably coupled between aphotodetector 902 and a plurality of signal processing paths (two shownas path A and path B). The signal conditioning circuitry 903 operates toamplify (and possible filter out unwanted noise components in) the scandata signal produced by the photodetector 902. The track and holdcircuitry 911 operates in response to timing signal SourceX-Enable,which defines time slots logically assigned to the photodetector 902, tooperably couple/decouple the output of the signal conditioning circuitry903 to the low pass filter 912 in response to the timing signalSourceX-Enable. When the SourceX-Enable signal is active (“track” mode),the output of the signal conditioning circuitry 903 is operably coupledto the low pass filter 912. When the SourceX-Enable signal is inactive(“hold” mode), the output of the signal conditioning circuitry 903 isoperably decoupled (e.g., electrically isolated) from the low passfilter 912. In addition, in the “hold” mod, the hold capacitor of thetrack and hold circuit 911 stores a sample of the data signals derivedfrom the photosensor 902 during the previous time slot assigned tophotosensor 902, and the voltage level at the output of track and holdcircuit 911 corresponds to the sampled voltage level stored on the holdcapacitor. The low pass filter 912 operates to smooth out or otherwisefilter the data signal supplied the track and hold circuit 911. Anillustrative implementation of the signal conditioning circuitry 903,track and hold circuit 911, and low pass filter circuitry is describedbelow with respect to FIGS. 13A and 13B.

The output of the low pass filter 912 is supplied to a plurality ofsignal processing paths, which process the output of the low pass filter912 in parallel. Each signal processing path includes: a firstderivative signal generation circuit 904 having a differentiator, lowpass filter and amplifier therein; a second derivative signal generationcircuit 906 having a differentiator therein; a first derivative signalthreshold-level generation circuit 905; and a zero crossing detector907, data gate 908, and binary-type A/D signal conversion circuitry 909.

The first derivative signal generation circuitry 904 in each respectivepath (labeled 904-A and 904-B in as shown) includes a differentiator,low pass filter and amplifier that generate a signal approximating thefirst derivative of the analog scan data signal (with unwanted noisecomponents removed). The low pass filter may be implemented with passiveelements (such as resistors, capacitors and inductors) or may beimplemented with active elements (such as an operational amplifier).Preferably, the low-pass filter implements one of a Butterworth-type,Chebsychev-type, MFTD-type, or elliptical-type low pass filteringtransfer function, which are well known in the filtering art. Details ofthe design of such filters is set forth in the book entitled “ElectricalFilter Design Handbook,” Third Edition, by A. Williams et al.,McGraw-Hill, 1996, herein incorporated by reference in its entirety. Anillustrative implementation of the first derivative signal generationcircuitry 904 for two different paths is described below with respect toFIG. 14.

The “first derivative signal” is supplied to second derivative signalgeneration circuit 906 and to first derivative threshold circuitry 905in the respective path. The second derivative signal generationcircuitry in each respective path (labeled 906-A and 906-B as shown)includes a differentiator that generates a signal approximating thesecond derivative of the scan data signal (with unwanted noisecomponents removed). An example of the second derivative signalgeneration circuitry is described below with respect to FIG. 15.

The “second derivative signal” is supplied to a zero crossing detector907 that produces output signal(s) (“zero crossing signal”) identifyingzero crossings in the second derivative signal. An illustrativeimplementation of the zero crossing detector in each respective path(labeled 907-A and 907-B) is described below with respect to FIG. 17.

The first derivative threshold circuitry in each respective path(labeled 905-A and 905-B) operates as a positive and negative peakdetector to provide output signals that indicate the approximate timeperiods when the positive and negative peaks of the first derivativesignal provided thereto exceed predetermined thresholds (i.e., apositive peak level PPL and a negative peak level NPL). An illustrativeimplementation of such first derivative threshold circuitry 905 for thetwo different paths is described below with respect to FIGS. 16A through16D.

In the absence of noise, the occurrence of each second derivativezero-crossing indicates that the “first derivative signal” is undergoinga (positive or negative) peak which corresponds to the point in the scandata signal where a signal level transition (e.g., indicative of atransition between a space and a bar in a bar code symbol) has occurred.However, in the real-world, noise is notorious for producing falsezero-crossing detections within the second derivative zero-crossingdetection circuit. To reduce the number of “falsely detected”zero-crossings produced by noise, data gating circuit 908 is provided,which functions to gate to the binary-type A/D signal conversioncircuitry 909, only detected second derivative zero-crossings whichoccur substantially concurrent to a positive or negative peak detectedin the “first derivative signal” (as identified by the outputs signalsof the first derivative threshold circuitry 905). An example of the datagate circuitry and binary-type A/D signal conversion circuitry isdescribed below with respect to FIG. 18.

The output of the binary-type A/D conversion circuitry 909 is a digitalscan data signal D₂ having first and second (i.e. binary) signal levelswhich correspond to the bars and spaces of the bar code symbol beingscanned. Thus, the digital scan data signal D₂ appears as a pulse-widthmodulated type signal as the first and second signal levels vary inproportion to the width of bars and spaces in the scanned bar codesymbol.

The digital scan data signal D₂ is supplied to digitizing circuitry,which converts the digital scan data signal D₂, associated with eachscanned bar code symbol, into a corresponding sequence of digital words(i.e. a sequence of digital count values) D₃. Notably, in the digitalword sequence D₃, each digital word represents the time lengthassociated with each first or second signal level in the correspondingdigital scan data signal D₂. Preferably, these digital count values arein a suitable digital format for use in carrying out various symboldecoding operations which, like the scanning pattern and volume of thepresent invention, will be determined primarily by the particularscanning application at hand. Reference is made to U.S. Pat. No.5,343,027 to Knowles, incorporated herein by reference, as it providestechnical details regarding the design and construction ofmicroelectronic digitizing circuits suitable for use in the laserscanner of the present invention.

Bar code symbol decoding circuitry (which is typically implemented witha programmed microprocessor/microcontroller) receive each digital wordsequence D₃ produced from the digitizing circuit, and subject it to oneor more bar code symbol decoding algorithms in order to determine whichbar code symbol is indicated (i.e. represented) by the digital wordsequence D₃, originally derived from corresponding scan data signal D₁detected by the photodetector associated therewith.

More than one multi-path data signal processor 901 may be employed by alaser scanning system in accordance with the present invention. Forexample, the bioptical laser scanning system of FIG. 7 may include threemulti-path data signal processors 901 as part of the analog signalprocessing circuitry operably coupled to the three photodetectors showntherein.

The operation of the multi-path scan data signal processor 901 isillustrated by the signal diagrams of FIGS. 12A through 12I. FIG. 12Adepicts the TDM scanning laser beam moving across bar and spaces of abar code. FIG. 12B 1 depicts the light level received at thephotodetector 902. FIG. 12B 2 depicts the light level received at thephotodetector at high resolution in time, which shows the effect of TDMlaser scanning. FIG. 12C 1 depicts the voltage level produced at theoutput node of the track and hold circuit at a high resolution in time,which shows the effect of TDM signal processing operations. FIG. 12C 2depicts the voltage level of the scan data signal produced at the outputof filter 912. And FIGS. 12D through 12I depict the processing performedin one of the respective paths of the multi-path scan data signalprocessor 901. Similar processing operations with different operationscharacteristics are performed in other paths of the multi-path scan datasignal processor 901.

More specifically, each signal processing path has different operationalcharacteristics (such as different cutoff frequencies in the filteringstages of the first and second derivative signal generation circuits ofthe respective paths, different gain characteristics in amplifier stagesof the first and second derivative signal generation circuits of therespective paths, and/or different positive and negative signalthresholds in the first derivative threshold circuitry of the respectivepaths). The varying operational characteristics of the paths areoptimized to provide different signal processing functions.

For example, the cut-off frequencies in the filtering stages of thefirst and second derivative signal generation circuits of the respectivepaths can vary such that different paths minimize the paper noiseoriginating from different focal zones of the system. Alternatively,such cut-off frequencies can vary such that one or more paths maximizethe scan resolution of the system (i.e., a path with higher cutofffrequencies may be able to detect high resolution bar code symbols)while other paths minimize paper noise (i.e., a path with lower cutofffrequencies will reject paper noise from a larger frequency band abovethe selected cutoff frequencies).

In another example, the gain characteristics in the amplifier stages ofthe first and second derivative signal generation circuits of the pathsand/or the positive and negative signal thresholds in the firstderivative threshold circuitry of the paths can vary such that one ormore paths maximize the scan resolution of the system (i.e., a path withhigher gain and/or smaller positive and negative signal thresholds maybe able to detect low bar code symbols) while other paths minimize papernoise (i.e., a path with lower gain and/or larger positive and negativesignal thresholds will reject paper noise that falls below suchthresholds).

The different signal processing functions of each path of the multi-pathscan data processor as described above are preferably performed inparallel. Alternatively, the processing along each path may be performedsequentially. In this case, a programmable microcomputer may beprogrammed to dynamically activate the processing of a given path basedupon the operation of the scanner (for example, based upon the focaldistance of the scanning plane from which the scan data signal isderived, which is described in detail in U.S. application Ser. No.(108-045USA000), or based upon results of previous scan processing ofthe system).

By virtue of this improved architecture, the multi-path scan data signalprocessor is able to identify signal level transitions (corresponding totransitions between a space and a bar in a bar code symbol) in the scandata signal over a diverse range of operating conditions (e.g.,operating conditions where paper noise is present in addition tooperating conditions requiring high resolution scanning, such as thereading of low contrast or high resolution bar code symbols), whichenables more reliable bar code reading over such diverse operatingconditions.

Signal Conditioning Circuitry/Track and Hold Circuitry/Low Pass Filter

FIGS. 13A and 13B illustrate an exemplary embodiment of the signalconditioning circuitry 903, track and hold circuit 911 and low passfilter 912 of FIG. 11C, which performs TDM signal processing operationsthat amplify and smooth out or otherwise filter the scan data signalproduced by the photodetector 902 to remove unwanted noise componentstherein over time slots defined by the SourceX_Enable timing signalprovided thereto. The circuitry 903 comprises, a number of subcomponentsarranged in a serial manner, namely: a high gain amplifier stage 1103, amultistage amplifier stage 1105, and a differential amplifier stage1107. The amplifier stages 1103, 1105 and 1109 amplify the voltage ofthe analog scan data signal produced by the photodetector 902 with gainsof 90, 3.0 and 7.1, respectively, to provide a total gain of about 1900.In addition, the amplifier stages preferably operate to filter outunwanted noise components in the scan data signal produced therefrom.The track and hold circuitry 911 operates in response to timing signalSourceX-Enable, which defines time slots logically assigned to thephotodetector 902, to operably couple/decouple the output of the signalconditioning circuitry 903 to the low pass filter 912 in response to thetiming signal SourceX-Enable. When the SourceX-Enable signal is active(“track” mode), the output of the signal conditioning circuitry 903 isoperably coupled to the low pass filter 912. When the SourceX-Enablesignal is inactive (“hold” mode), the output of the signal conditioningcircuitry 903 is operably decoupled (e.g., electrically isolated) fromthe low pass filter 912. In addition, in the “hold” mode, the holdcapacitor of the track and hold circuit 911 stores a sample of the datasignals derived from the photosensor 902 during the previous time slotassigned to photosensor 902, and the voltage level at the output oftrack and hold circuit 911 corresponds to the sampled voltage levelstored on the hold capacitor. The low pass filter circuitry 912 operatesto filter out unwanted noise in the amplified signal produced by theamplifier stages 1103, 1105 and 1109. The low pass filter 912 shown isdesigned to filter out unwanted high frequency noise (e.g., noise whichlies above the expected maximum signal frequency of 540 kHz).

The First Derivative Signal Generation Circuitry

FIG. 14 illustrates an exemplary implementation of the first derivativesignal generation circuitry 904, which is suitable for use in the twodifferent paths of the scan data signal processor of FIG. 9C. As shownin FIG. 14, the first derivative signal generation circuitry 904includes a number of subcomponents arranged in a serial manner thatprocess the analog scan data signal produced by the signal conditioningcircuitry 903, namely: a differentiator stage 1201, a low-pass filter(LPF) stage 1203, and an amplifier stage 1205.

The differentiator stage 1201 generates an signal whose voltage level isproportional to the first derivative of the analog scan data signal forthose frequencies less than the cutoff frequency of the differentiatorstage 1201, which is set by the values of R43 and C32, respectively, andcan be approximated by the expression:${f_{c} = \frac{1}{2*\pi*R\quad 43*C\quad 32}},$which is approximately 3.226 MHz for the circuit elements shown.

The low pass filter stage 1203 operates to filter out unwanted noise inthe output signal produced by the differentiator stage 1201. The 3 dBcutoff frequency of the low pass filter shown (which is a maximally flatButterworth type filter) is set by the values of L5 and C36,respectively, and can be approximated by the expression:${f_{c} = \frac{1}{2*\pi*\sqrt{L\quad 5*C\quad 36}}},$which is approximately 650 kHz for the circuit elements shown.

The amplifier stage 1205 operates to amplify the voltage levels of theoutput signal produced by the LPF stage for frequencies in apredetermined frequency band. More specifically, for frequencies betweenf₁ and f₂, the amplifier produces a gain that is approximatelyproportional to R60/R54 (which is approximately 6.5 for the circuitelements shown) where: ${f_{1} = \frac{1}{2*\pi*R\quad 54*C\quad 39}},$which is approximately 3 kHz for the circuit elements shown.${f_{2} = \frac{1}{2*\pi*R\quad 60*C\quad 43}},$which is approximately 2 MHz for the circuit elements shown.Outside the predetermined frequency band between f₁ and f₂, theamplifier stage 1205 attenuates such frequency components.

It should be noted that although the first derivative signal generationcircuitry of the two paths (labeled 904-A and 904-B in FIG. 11C) share acommon function—to generate a signal approximating the first derivativeof the analog scan data signal—they may have different operationalcharacteristics that are optimized for bar code scanning and reading indiverse operating conditions.

For example, the cut-off frequencies in the differentiator stage 1201,the LPF stage 1203 and the amplifier stage 1205 of the first derivativesignal generation circuits of the respective paths (labeled 904-A and904-B) can vary (by selecting different values for the appropriatecircuit elements as set forth above) such that different paths minimizethe paper noise originating from different focal zones of the system.Techniques for selecting the appropriate cutoff frequencies thatcorrespond to the different focal zones of the laser scanning system aredescribed in detail in U.S. patent application Ser. No. (108-045USA000),commonly assigned to the assignee of the present application,incorporated by reference above in its entirety. Alternatively, suchcut-off frequencies can vary such that one or more paths maximize thescan resolution of the system (i.e., a path with higher cutofffrequencies may be able to detect high resolution bar code symbols)while other paths minimize paper noise (i.e., a path with lower cutofffrequencies will reject paper noise from a larger frequency band abovethe selected cutoff frequencies).

In another example, the gain characteristics in the amplifier stage 1205of the first derivative signal generation circuits of the respectivepaths (labeled 904-A and 904B) can vary such that one path maximizes thescan resolution of the system (i.e., a path with higher gain may be ableto detect low bar code symbols) while the other path minimize papernoise (i.e., a path with lower gain will reject paper noise that mighttrigger scan errors when amplified by the high gain path).

The Second Derivative Signal Generation Circuitry

FIG. 15 illustrate an exemplary implementation of the second derivativesignal generation circuitry 906, which is suitable for use in the twodifferent paths of the scan data signal processor of FIG. 11C. As shownin FIG. 15, the second derivative signal generation circuitry 906includes a differentiator stage 1301 that generates a signal whosevoltage level is proportional to the derivative of the first derivativesignal produced by the first derivative generation circuitry 904 (thusproportional to the second derivative of the analog scan data signalproduced by the signal conditioning circuitry 903) for frequencies in apredetermined frequency band. More specifically, the differentiatorstage 1301 operates substantially as a differentiator (producing asignal whose voltage level is proportional to the derivative of thefirst derivative signal produced by the first derivative generationcircuitry 904) for frequencies less than f₁ where:${f_{1} = \frac{1}{2*\pi*R\quad 62*C\quad 48}},$which is approximately 884 kHz for the circuit elements shown.Moreover, the feedback elements of the differentiator stage 1301 operatesubstantially as a low pass filter with a 3 dB cutoff frequency which isset by the values of R65 and C49, respectively, and can be approximatedby the expression: ${f_{c} = \frac{1}{2*\pi*R\quad 65*C\quad 49}},$which is approximately 2.15 Mhz for the circuit elements shown.For frequencies above this predetermined 3 dB cutoff frequency f_(c),the differentiator stage 1301 attenuates such frequency components.

It should be noted that although the second derivative signal generationcircuitry of the two paths (labeled 906-A and 906-B in FIG. 11C) share acommon function—to generate a signal approximating the second derivativeof the analog scan data signal—they may have different operationalcharacteristics that are optimized for bar code scanning and reading indiverse operating conditions.

For example, the cut-off frequencies in the differentiator stage 1301 ofthe second derivative signal generation circuits of the respective paths(labeled 906-A and 906-B) can vary (by selecting different values forthe appropriate circuit elements as set forth above) such that differentpaths minimize the paper noise originating from different focal zones ofthe system. Techniques for selecting the appropriate cutoff frequenciesthat correspond to the different focal zones of the laser scanningsystem are described in detail in U.S. patent application Ser. No.(108-045USA000), commonly assigned to the assignee of the presentapplication, incorporated by reference above in its entirety.Alternatively, such cut-off frequencies can vary such that one or morepaths maximize the scan resolution of the system (i.e., a path withhigher cutoff frequencies may be able to detect high resolution bar codesymbols) while other paths minimize paper noise (i.e., a path with lowercutoff frequencies will reject paper noise from a larger frequency bandabove the selected cutoff frequencies).

Zero Crossing Detector

FIG. 17 illustrates an exemplary implementation of a zero crossingdetector 907, which is suitable for use in the two different paths ofthe scan data signal processor of FIG. 11C. As shown in FIG. 15, thezero-crossing detector 907 includes a comparator circuit that comparesthe second derivative signal produced from the second derivativegeneration circuit in its respective path with a zero voltage reference(i.e. the AC ground level) provided by the zero reference signalgenerator, in order to detect the occurrence of each zero-crossing inthe second derivative signal, and provide output signals (ZC_1 and ZC_2signals) identifying zero crossings in the second derivative signal.

First Derivative Signal Threshold Level Generation Circuit

FIGS. 16A through 16D illustrate exemplary implementation of the firstderivative signal threshold circuitry 905, which is suitable for use inthe two different paths of the scan data signal processor of FIG. 11C.As shown in FIGS. 16A through 16D, the first derivative signal thresholdcircuitry 905 includes an amplifier stage 1401 that amplifies thevoltage levels of the first derivative signal produced by the firstderivative signal generation circuitry 904, positive and negative peakdetectors 1403 and 1405, and a comparator stage 1407 that generatesoutput signals (e.g., the Upper_Threshold Signal and Lower_ThresholdSignal) that indicate the time period when the positive and negativepeaks of the amplified first derivative signal produced by the amplifierstage exceed predetermined thresholds (i.e., a positive peak level PPLand a negative peak level NPL). Preferably, the positive peak level PPLand negative peak level NPL are dynamic thresholds (e.g., these levelschange as the amplified analog signal changes over time) based upon a DCbias level and a percentage (portion) of the amplified first derivativesignal produced by the amplifier stage 1401. In the illustrativeembodiment shown in FIGS. 16A through 16D, capacitors C16 and C18 areconfigured as peak detectors (with a decay time constant proportional tothe values of R14/C16 and R19/C18, respectively); and the positive peaklevel PPL is set by the resistance values of the resistor networkR16,R17,R18 and R_(U) _(—) _(BIAS), while the negative peak level NPL isset by the values of the resistor network R21,R22,R23 and R_(L) _(—)_(BIAS).

It should be noted that although the first derivative signal thresholdcircuitry of the two paths (labeled 905-A and 905-B in FIG. 11C) share acommon function—to generate output signals that indicate the time periodwhen the positive and negative peaks of the amplified first derivativesignal exceed predetermined thresholds—they may have differentoperational characteristics that are optimized for bar code scanning andreading in diverse operating conditions.

For example, the positive and negative peak levels in the positive andnegative peak detectors 1403 and 1405, respectively, (which are set bythe resistance values of the resistor networks therein) can vary suchthat one path maximizes the scan resolution of the system (i.e., a pathwith lower positive peak and negative peak level may be able to detectlow bar code symbols) while the other path minimize paper noise (i.e., apath with a higher positive peak and negative peak level will rejectpaper noise that that falls below such thresholds).

For example, the positive and negative peak detectors 1403 and 1405 inthe first derivative signal threshold circuitry 905-A of the first pathA may utilize a 91 kilo-ohm resistor for R_(U) _(—) _(BIAS) and R_(L)_(—) _(BIAS) of FIGS. 16A through 16D. Such resistor values produce adynamic PPL threshold which approximates 2.079 mV DC bias level plus 24%of the amplified first derivative signal, and produce a dynamic NPLthreshold which approximates a 1.921 mV DC bias level less 24% of theamplified first derivative signal. In another example, the positive andnegative peak detectors 1403 and 1405 in the first derivative signalthreshold circuitry 905-B of the second path B may utilize a 20 kilo-ohmresistor for R_(U) _(—) _(BIAS) and R_(L) _(—) _(BIAS) of FIGS. 16Athrough 16D. Such resistor values produce a dynamic PPL threshold whichapproximates a 2.316 mV DC bias level plus 21% of the amplified firstderivative signal, and produce a dynamic NPL threshold whichapproximates 1.684 mV DC bias level less 21% of the amplified firstderivative signal. Note that path A has “lower” positive peak andnegative peak levels—it may be able to detect high resolution bar codesymbols than path B. While path B has “higher” positive peak andnegative peak levels—it will reject paper noise that might trigger scanerrors in the path A).

Data Gating Circuitry and 1-Bit A/D Conversion Circuitry

FIG. 18 illustrates an exemplary implementation of the data gatingcircuitry 908 and 1-Bit A/D conversion circuitry 909, which is suitablefor use in the two different paths of the scan data signal processor ofFIG. 11C. In each respective path, the data gating circuit 908 functionsto gate to the binary-type A/D signal conversion circuitry 909, onlydetected second derivative zero-crossings (identified by the outputssignals ZC_1 and ZC_2 of the zero crossing detector 907 in therespective path) which occur substantially concurrent to a positive ornegative peaks detected in the “first derivative signal” (as identifiedby the output signals—Upper_Threshold and Lower_Threshold—of the firstderivative threshold circuitry 905). As shown in FIG. 18, the data gatecircuit 908 and the 1 bit D/A conversion circuitry 909 in each path isrealized by four NAND gates (labeled 601 through 1604) configured as aset/reset latch circuit. The operation of the data gating circuitry and1 bit D/A conversion circuitry of FIG. 18 is illustrated in the signalplot of FIG. 12I.

Having described illustrative embodiments of the present invention, itis understood that there a number of alternative ways to practice thepresent invention. Several different modes for carrying out the presentinvention will be described below.

For example, rather than using “analog-type” circuit technology forrealizing the signal processing subcomponents of the TDM-enabledmulti-path scan data signal processor (e.g., the differentiators,low-pass filter, amplifiers, peak detectors, data gate, etc.), it isunderstood that the scan data signal processing method and apparatus ofthe present invention can be implemented using digital signal processingtechniques carried out either within a programmed microcomputer or usingone or more custom or commercially available digital signal processing(DSP) chips known in the digital signal processing art. Examples ofsystem architectures utilizing digital signal processing technology forbar code detection and decoding is described above with respect to FIGS.9B and 9C.

As illustrated in FIG. 19A, when carrying out a digital implementationof the scan data signal processor of the present invention, the analogscan data signal D₁ is provided to signal conditioning circuitry 1703(which amplifies and filters the signal to remove unwanted noisecomponents as described above), whose output is provided toanalog-to-digital conversion circuitry 1705. The sampling function (andpossibly the conversion function) of analog-to-digital conversioncircuitry 1705 in selectively enabled in response to timing signalSourceX-Enable that defines time slots logically assigned to thephotodetector 1701 to thereby enable sampling and conversion of theanalog scan data signals produced by the signal conditioning circuitry1703 to digital scan data signals during such time slots, and disablesuch sampling/conversion operations during time periods outside suchtime slots. During such time slots, the analog-to-digital conversioncircuitry 1705 samples the conditioned analog scan data signals at asampling frequency at least two times the highest frequency componentexpected in the analog scan data signal, in accordance with the wellknown Nyquist criteria, and quantizes each time-sampled scan data signalvalue into a discrete signal level using a suitable length numberrepresentation (e.g. 8 bits) to produce a discrete scan data signal. Asuitable quantization level can be selected in view of expected noiselevels in the signal. Thereafter, the discrete scan data signal isprocessed by the programmed processor (e.g., a digital signal processor1707 and associated memory 1709 as shown) to generate a sequence ofdigital words (i.e. a sequence of digital count values) D₃, eachrepresenting the time length associated with the signal leveltransitions in the corresponding digital scan data signal as describedabove. Preferably, these digital count values are in a suitable digitalformat for use in carrying out various symbol decoding operations which,like the scanning pattern and volume of the present invention, will bedetermined primarily by the particular scanning application at hand.

FIGS. 19B through 19D illustrate exemplary digital implementations ofthe TDM-enabled multi-path scan data processing according to the presentinvention. The digital signal processing operations therein arepreferably carried out on the discrete scan data signal levels generatedby the A/D converter 1705 during the time periods logically assigned tothe photodetector 1701 and stored in the memory 1709 of FIG. 19A.

FIG. 19B illustrates exemplary digital signal processing operations thatidentify a data frame (e.g., a portion of the discrete scan data signallevels stored in memory 1709) that potentially represents a bar codesymbol (block 1723) and stores the data frame in a working buffer (block1725). Signal processing techniques that identify a data frame (withinthe discrete signal levels stored in the memory 1709) that potentiallyrepresents a bar code symbol (block 1723) are well know in the art.

FIG. 19C illustrates exemplary digital signal processing operations thatcarry out multi-path scan data signal processing according to thepresent invention. More specifically, in block 1727, a data frame isread from the working buffer. Preferably, the data frame read from theworking buffer in block 1727 was stored therein in block 1725 of FIG.19B. Alternatively, the data frame may be a block of the discrete scandata signals levels generated by the A/D converter 1705 and stored inmemory 1709 of FIG. 19A (or discrete scan data signals derivedtherefrom). The data values of the data frame are then processed by asequence of signal processing blocks (blocks 1729, 1731-1745 and1751-1765).

In block 1729, such data values are optionally interpolated (orsub-sampled). Interpolation increases the effective sampling rate of thesystem by adding data values that are derived from existing data values.Interpolation is a technique well known in the digital signal processingarts, and is discussed in great detail in Russ, “Image ProcessingHandbook,” Third Edition, IEEE Press, 1999, pg. 219-220, hereinincorporated by reference in its entirety. Sub-sampling (or decimation)decreases the effective sampling rate of the system. Sub-sampling istypically accomplished by averaging data values. Sub-sampling is atechnique well known in the digital signal processing arts, and isdiscussed in great detail in Russ, “Image Processing Handbook,” ThirdEdition, IEEE Press, 1999, pg. 166-174, herein incorporated by referencein its entirety. The resulting block of data values are provided to atleast two processing paths (for example, two paths A and B as shown).The different digital signal processing functions of each path arepreferably performed in parallel (for example, by separated threads in amulti-threaded processing system or by separate processors in amulti-processor system). Alternatively, the processing along each pathmay be performed sequentially.

In each respective processing path, the block of data values are subjectto a digital low pass filter (blocks 1731 and blocks 1753) that filterout unwanted noise. Such digital low-pass filters preferably implementone of a Butterworth-type, Chebsychev-type, MFTD-type, orelliptical-type low pass filtering transfer function, which are wellknown in the filtering art. Details of the design of such digitalfilters is set forth in the book entitled “Electrical Filter DesignHandbook,” Third Edition, by A. Williams et al. McGraw-Hill, 1996,incorporated by reference above in its entirety. The output of thedigital low pass filter (blocks 1731, 1751) is supplied to a firstderivative processing function (blocks 1733, and 1753) whichdifferentiate the filtered digital scan data signals supplied thereto.The output of the first derivative processing function (blocks 1733,1753) is normalized (blocks 1734, 1754) and supplied to a firstderivative thresholding function (blocks 1739 and 1759) and a secondderivative processing function (blocks 1735 and 1755).

The second derivative processing function (blocks 1735, 1755)differentiates the data supplied thereto to generate data representingthe second derivative of the data values read from the working buffer.Such data is supplied to a zero crossing detector function (blocks 1737,1757), which produces output data (“zero crossing data”) identifyingzero crossings in the second derivative data generated by the secondderivative function (blocks 1735, 1755).

The first derivative thresholding function (blocks 1739, 1759) operatesas a positive and negative peak detector to provide output data thatidentifies time periods when the positive and negative peaks of the datasupplied thereto exceed predetermined thresholds (i.e., a positive peaklevel PPL and a negative peak level NPL). Preferably, the positive peaklevel PPL and negative peak level NPL are dynamic thresholds (e.g.,these levels change as the digital scan data values read from theworking buffer change over time) based upon a predetermined digitalvalue and a percentage (portion) of the corresponding normalized firstderivative signal supplied thereto.

The data output of the zero crossing detector function (blocks 1737,1757) and the first derivative thresholding function (blocks 1739, 1759)are supplied to a data gate function (blocks 1741, 1761), whichfunctions to output only zero crossing data which corresponds todetected zero-crossings which occur substantially concurrent with thepositive or negative peaks detected in the normalized first derivativedata (as identified by the output data of the first derivative thresholdfunction). Thereafter, the data output by the data gate function (whichrepresents a discrete binary-level scan data signal) is supplied to abar length function (blocks 1743, 1763), which produce a digital “time”count value for each of the first and second signal levels in thediscrete binary scan data signal. Such digital count values form asequence of digital word D₃, each representing the time lengthassociated with the signal level transitions in the correspondingdigital scan data signal as described above. These digital words arestored in an output buffer (blocks 1745, 1765), for supply to aprogrammed decoder for decoding the scan data signal and producingsymbol character data string representative of the correspondinglaser-scanned bar code symbol. Alternatively, the generated discretebinary-level scan data signal can be converted back into acontinuous-type binary-level scan data signal so that it may be“digitized” using a digital signal processor of the type taught in U.S.Pat. No. 5,828,049, incorporated herein by reference.

Each digital signal processing path has different operationalcharacteristics (such as different cutoff frequencies in the low passfilters (blocks 1731 and 1751) and/or different positive and negativesignal thresholds in the first derivative threshold function (blocks1739, 1759) of the respective paths). The varying operationalcharacteristics of the paths are optimized to provide different digitalsignal processing functions.

For example, the cut-off frequencies in the low pass filters (blocks1731 and 1751) of the respective paths can vary such that differentpaths minimize the paper noise originating from different focal zones ofthe system. Alternatively, such cut-off frequencies can such that varysuch that one or more paths maximize the scan resolution of the system(i.e., a path with higher cutoff frequencies may be able to detect highresolution bar code symbols) while other paths minimize paper noise(i.e., a path with lower cutoff frequencies will reject paper noise froma larger frequency band above the selected cutoff frequencies).

In another example, the positive and negative signal thresholds in thefirst derivative threshold functions (blocks 1739, 1759) of therespective paths can vary such that one or more paths maximize the scanresolution of the system (i.e., a path with “smaller” positive andnegative signal thresholds may be able to detect low contrast bar codesymbols) while other paths minimize paper noise (i.e., a path with a“larger” positive and negative signal thresholds will reject paper noisethat falls below such thresholds.

The different digital signal processing functions of each path asdescribed above are preferably performed in parallel (for example, byseparated threads in a multi-threaded processing system or by separateprocessors in a multi-processor system). Alternatively, the processingalong each path may be performed sequentially. In this case, theprogrammable microcomputer (e.g., digital signal processing system) maybe programmed to dynamically activate the processing of a given pathbased upon the operation of the scanner (for example, based upon thefocal distance of the scanning plane from which the scan data signal isderived, which is described in detail in U.S. application Ser. No.(108-045USA000), or based upon results of previous scan processing ofthe system.

FIG. 19D illustrates alternative digital signal processing operationsthat carry out multi-path scan data signal processing according to thepresent invention. More specifically, in block 1771, a data frame isread from the working buffer. Preferably, the data frame read from theworking buffer in block 1727 was stored therein in block 1725 of FIG.19B. Alternatively, the data frame may be a block of the discrete scandata signals levels generated by the A/D converter 1705 and stored inmemory 1709 of FIG. 19A (or discrete scan data signals derivedtherefrom). In block 1773, such data values are optionally interpolated(or sub-sampled). Interpolation increases the effective sampling rate ofthe system by adding data values that are derived from existing datavalues.

In block 1775, the resulting block of data values are subject to adigital low pass filter that filters out unwanted noise. Such digitallow-pass filter preferably implements one of a Butterworth-type,Chebsychev-type, MFTD-type, or elliptical-type low pass filteringtransfer function, which are well known in the filtering art. Details ofthe design of such digital filters is set forth in the book entitled“Electrical Filter Design Handbook,” Third Edition, by A. Williams etal. McGraw-Hill, 1996, incorporated by reference above in its entirety.The output of the digital low pass filter (block 1775) is supplied to afirst derivative processing function (block 1777) which differentiatesthe filtered digital scan data signals supplied thereto. The output ofthe first derivative processing function (block 1777) is normalized(block 1779) and supplied to a second derivative processing function(block 1781).

The second derivative processing function (block 1781) differentiatesthe data supplied thereto to generate data representing the secondderivative of the data values read from the working buffer. Such data issupplied to a zero crossing detector function (block 1783), whichproduces output data (“zero crossing data”) identifying zero crossingsin the second derivative data generated by the second derivativefunction.

The normalized output of the first derivative processing function (block1779) is also supplied to at least one processing sub-path (for example,sub-path A as shown). In the illustrative embodiment shown in FIG. 19D,the execution of the signal processing of the second sub-path B iscontingent upon a status condition of the working buffer (e.g., whetherit has (or has not) received another full data frame. Alternatively, thedifferent digital signal processing functions of each sub-path may beperformed in parallel (for example, by separated threads in amulti-threaded processing system or by separate processors in amulti-processor system).

Each processing sub-path includes a first derivative thresholdingfunction (blocks 1785, 1795), which operates as a positive and negativepeak detector to provide output data that identifies time periods whenthe positive and negative peaks of the data supplied thereto exceedpredetermined thresholds (i.e., a positive peak level PPL and a negativepeak level NPL). Preferably, the positive peak level PPL and negativepeak level NPL are dynamic thresholds (e.g., these levels change as thedigital scan data values read from the working buffer change over time)based upon a predetermined digital value and a percentage (portion) ofthe corresponding normalized first derivative signal supplied thereto.

The data output of the zero crossing detector function (block 1783) andthe first derivative thresholding function of the respective path (block1785, 1795) are supplied to a data gate function (blocks 1787, 1797),which functions to output only zero crossing data which corresponds todetected zero-crossings which occur substantially concurrent with thepositive or negative peaks detected in the normalized first derivativedata (as identified by the output data of the first derivative thresholdfunction). Thereafter, the data output by the data gate function (whichrepresents a discrete binary-level scan data signal) is supplied to abar length function (blocks 1789, 1798), which produce a digital “time”count value for each of the first and second signal levels in thediscrete binary scan data signal. Such digital count values form asequence of digital word D₃, each representing the time lengthassociated with the signal level transitions in the correspondingdigital scan data signal as described above. These digital words arestored in an output buffer (blocks 1791, 1799), for supply to aprogrammed decoder for decoding the scan data signal and producingsymbol character data string representative of the correspondinglaser-scanned bar code symbol. Alternatively, the generated discretebinary-level scan data signal can be converted back into acontinuous-type binary-level scan data signal so that it may be“digitized” using a digital signal processor of the type taught in U.S.Pat. No. 5,828,049, incorporated herein by reference.

Each digital signal processing sub-path of FIG. 19D has differentoperational characteristics (such as different positive and negativesignal thresholds in the first derivative threshold function (blocks1785, 1795) of the respective sub-paths). The varying operationalcharacteristics of the sub-paths are optimized to provide differentdigital signal processing functions.

For example, the positive and negative signal thresholds in the firstderivative threshold functions (blocks 1785, 1795) of the respectivesub-paths can vary such that one or more sub-paths maximize the scanresolution of the system (i.e., a sub-path with “smaller” positive andnegative signal thresholds may be able to detect low bar code symbols)while other sub-paths minimize paper noise (i.e., a sub-path with a“larger” positive and negative signal thresholds will reject paper noisethat falls below such thresholds.

Note that the illustrative embodiments set forth above provide aTDM-enabled multi-path scan data signal processor with two signalprocessing paths (or sub-paths) with different operationalcharacteristics. It is contemplated that the TDM-enabled multi-path scandata signal processor of the present invention includes more than twosignal processing paths (or sub-paths) with different operationalcharacteristics as described above.

Advantageously, the improved TDM-enabled scan data signal processor ofthe present invention has an improved signal-to-noise ratio (SNR) anddynamic range, which effectively increases the length of each focal zonein the laser scanning system. This allows the system designer to providemore overlap between adjacent focal zones or produce a laser scanningsystem with a larger overall depth of field. In addition, it produces alaser scanning system capable of scanning/resolving bar code symbolshaving narrower element widths and/or printed on substrates whose normalvector is disposed at large angles from the projection axis of laserscanning system.

While the various embodiments of the laser scanner hereof have beendescribed in connection with linear (1-D) bar code symbol scanningapplications, it should be clear, however, that the scanning apparatusand methods of the present invention are equally suited for scanning 2-Dbar code symbols, as well as alphanumeric characters (e.g. textualinformation) in optical character recognition (OCR) applications, aswell as scanning graphical images in graphical scanning arts.

Several modifications to the illustrative embodiments have beendescribed above. It is understood, however, that various othermodifications to the illustrative embodiment of the present inventionwill readily occur to persons with ordinary skill in the art. All suchmodifications and variations are deemed to be within the scope andspirit of the present invention as defined by the accompanying Claims toInvention.

1-50. (canceled)
 51. A bar code symbol scanning system comprising: atleast one laser light source for generating a plurality of laserscanning beams; optical elements for projecting said plurality of laserscanning beams into a scanning volume at different orientations forscanning one or more bar code symbols on objects within the scanningvolume, collecting light reflections from said laser scanning beams, anddirecting these light reflections to a plurality of photosensorscorresponding in number to the plurality of laser scanning beams andproducing electrical signals corresponding to light reflections from thescanned bar code symbols; signal processing circuitry operably, coupledto the output of said photosensors, for processing said electricalsignals and detecting and decoding the bar code symbols scanned by thelaser scanning beams; timing signal generation circuitry for generatingtiming signals that correspond to successive non-overlapping time slots,wherein each said laser scanning beam and corresponding photosensor islogically assigned to one of said successive non-overlapping time slots;a laser light source control mechanism for operating, during a giventime slot, in response to the timing signals generated by said timingsignal generation circuitry, so as to selectively enable one said laserscanning beam, logically assigned to the given time slot, to begenerated and/or projected into said scanning volume; and a signalprocessing control mechanism for operating, during the given time slot,in response to the timing signals generated by said timing signalgeneration circuitry, so as to selectively enable one said photosensor,logically assigned to the given time slot, to be operably coupled tosaid signal processing circuitry.
 52. The bar code symbol scanningsystem of claim 51, wherein a subset of said multiple scanning beams aresubstantially coincident to one another.
 53. The bar code symbolscanning system of claim 51, wherein said at least one light sourcecomprises a plurality of visible laser diodes, and wherein said laserlight source control mechanism selectively disables generation of thescanning laser beam by a given visible laser diode by controlling powersupplied to the given visible laser diode.
 54. The bar code symbolscanning system of claim 51, wherein each pair of non-overlapping timeslots in bounded by a null period; wherein said laser light sourcecontrol mechanism that operates, in response to the timing signalsgenerated by said timing signal generation circuitry, to disablegeneration and/or projection of said plurality of laser scanning beamsinto said scanning volume during each null period; and wherein thesignal processing control mechanism operates, in response to the timingsignals generated by said timing signal generation circuitry, so as tooperably decouple said plurality of photosensors from said signalprocessing circuitry during each null period.
 55. The bar code symbolscanning system of claim 51, including at least a horizontal housingportion with a first scanning window disposed therein, wherein saidmultiple laser scanning beams are projected at different orientationsthrough said first scanning window into a scanning volume disposed abovethe horizontal window.
 56. The bar code symbol scanning system of claim51, including at least a horizontal housing portion with a firstscanning window disposed therein and a vertical housing portion with asecond scanning window disposed therein, wherein said multiple laserscanning beams are projected at different orientations through saidfirst and second scanning windows into a scanning volume disposedadjacent the first and second scanning windows.
 57. The bar code symbolscanning system of claim 51, wherein said timing signal generatorincludes an oscillator and multi-state counter.
 58. The bar code symbolscanning system of claim 51, wherein said at least one laser lightsource includes a plurality of visible laser diodes, and said laserlight source control mechanism selectively disables said generation ofthe laser scanning beam by a given visible laser diode by controllingsupply of power to the given visible laser diode.
 59. The bar codesymbol scanning system of claim 51, wherein said at least one laserlight source includes a visible laser diode, and said laser light sourcecontrol mechanism selectively disables projection of the laser scanningbeam generated by the visible laser diode with an optical shutter. 60.The bar code symbol scanning system of claim 51, wherein said at leastone laser light source includes a visible laser diode, and said laserlight source control mechanism selectively disables projection of thelaser scanning beam generated by the visible laser diode with a beamdeflector.
 61. The bar code symbol scanning system of claim 51, whereinsaid signal processing control mechanism includes multiplexing circuitrycoupled between said plurality of photosensors and said signalprocessing circuitry, and wherein said multiplexing circuitry iscontrolled to selectively couple signal processing circuitry to a givenphotosensors during a time slot corresponding thereto.
 62. The bar codesymbol scanning system of claim 61, wherein said signal processingcircuitry includes shared analog to digital signal conversion circuitrythat processes signals derived from any one of said plurality ofphotosensors when operably coupled thereto via said multiplexingcircuitry.
 63. The bar code symbol scanning system of claim 62, whereinsaid signal processing control mechanism selectively enables said sharedanalog to digital signal conversion circuitry during time slots when anyone of said plurality of photosensors is operably coupled hereto viasaid multiplexing circuitry.
 64. The bar code symbol scanning system ofclaim 51, wherein said signal processing circuitry includes a pluralityof analog to digital signal converters each processing signals derivedfrom a unique one of said plurality of photosensors, and wherein saidsignal processing control mechanism selectively enables one of saidplurality of analog to digital signal converters during a time slotcorresponding thereto.
 65. The bar code symbol scanning system of claim51, wherein said laser light source control mechanism turns on a givenlaser scanning beam by operating a visible laser diode module thatproduces such laser scanning beam at an optical power level much greaterthan its threshold optical power level.
 66. The bar code symbol scanningsystem of claim 51, said laser light source control mechanism turnssubstantially off a given laser scanning beam by operating a visiblelaser diode module that produces such laser scanning beam at an opticalpower level less than its threshold optical power level.
 67. The barcode symbol scanning system of claim 51, said laser light source controlmechanism turns substantially off a given laser scanning beam byoperating a visible laser diode module that produces such laser scanningbeam at an optical power level near its threshold optical power level,thereby enabling quick turn on of the visible laser diode module. 68.The bar code symbol scanning system of claim 51, wherein said laserlight source control mechanism turns on a given laser scanning beam bysupplying current to a visible laser diode module that produces suchlaser scanning beam a current level much greater than threshold currentfor said visible laser diode module.
 69. The bar code symbol scanningsystem of claim 68, wherein said laser light source control mechanismcontrols said current level provided to said visible laser diode moduleby modulating a dynamic current source.
 70. The bar code symbol scanningsystem of claim 69, wherein said dynamic current source is directlycoupled to said visible laser diode module.
 71. The bar code symbolscanning system of claim 51, wherein said laser light source controlmechanism turns substantially off a given laser scanning beam bysupplying current to a visible laser diode module that produces suchlaser scanning beam at a current level near or less than thresholdcurrent for said visible laser diode module.
 72. The bar code symbolscanning system of claim 71, wherein said laser light source controlmechanism controls current level provided to said visible laser diodemodule by modulating a dynamic current source.
 73. The bar code symbolscanning system of claim 72, wherein said dynamic current source isdirectly coupled to said visible laser diode module.
 74. The bar codesymbol scanning system of claim 51, wherein frequency of time slotslogically assigned to a given laser scanning beam and correspondingphotosensor is greater than at least two times the highest frequencycomponent expected in the scan data signal received at said photosensor.75. The bar code symbol scanning system of claim 51, wherein time slotslogically assigned to a given laser scanning beam and correspondingphotosensor correspond to scanning planes generated by the given laserscanning beam during revolution of at least one rotating polygonalmirror.
 76. The bar code symbol scanning system of claim 51, whereintime slots logically assigned to a given laser scanning beam andcorresponding photosensor correspond to scanning plane groups generatedby the given laser scanning beam during revolution of at least onerotating polygonal mirror.